How Browsers Work:Behind the Scenes of Modern Web Browser(II)

Chapter 4

Render tree construction

While the DOM tree is being constructed, the browser constructs another tree, the render tree. This tree is of visual elements in the order in which they will be displayed. It is the visual representation of the document. The purpose of this tree is to enable painting the contents in their correct order.

Firefox calls the elements in the render tree "frames". Webkit uses the term renderer or render object. 
A renderer knows how to layout and paint itself and its children. 
Webkits RenderObject class, the base class of the renderers, has the following definition:

classRenderObject{virtualvoid layout();virtualvoid paint(PaintInfo);virtualvoid rect repaintRect();Node* node;//the DOM nodeRenderStyle* style;// the computed styleRenderLayer* containgLayer;//the containing z-index layer}

 

Each renderer represents a rectangular area usually corresponding to the node's CSS box, as described by the CSS2 spec. It contains geometric information like width, height and position. 
The box type is affected by the "display" style attribute that is relevant for the node (see the style computation section). Here is Webkit code for deciding what type of renderer should be created for a DOM node, according to the display attribute.

RenderObject*RenderObject::createObject(Node* node,RenderStyle* style){Document* doc = node->document();RenderArena* arena = doc->renderArena();...RenderObject* o =0;switch(style->display()){case NONE:break;case INLINE: o =new(arena)RenderInline(node);break;case BLOCK: o =new(arena)RenderBlock(node);break;case INLINE_BLOCK: o =new(arena)RenderBlock(node);break;case LIST_ITEM: o =new(arena)RenderListItem(node);break;...}return o;} The element type is also considered, for example form controls and tables have special frames. 
In Webkit if an element wants to create a special renderer it will override the createRenderer method. The renderers points to style objects that contains the non geometric information.

 

4.1The render tree relation to the DOM tree

The renderers correspond to the DOM elements, but the relation is not one to one. Non visual DOM elements will not be inserted in the render tree. An example is the "head" element. Also elements whose display attribute was assigned to "none" will not appear in the tree (elements with "hidden" visibility attribute will appear in the tree).

 

There are DOM elements which correspond to several visual objects. These are usually elements with complex structure that cannot be described by a single rectangle. For example, the "select" element has 3 renderers - one for the display area, one for the drop down list box and one for the button. Also when text is broken into multiple lines because the width is not sufficient for one line, the new lines will be added as extra renderers. 
Another example of several renderers is broken HTML. According to CSS spec an inline element must contain either only block element or only inline elements. In case of mixed content, anonymous block renderers will be created to wrap the inline elements.

Some render objects correspond to a DOM node but not in the same place in the tree. Floats and absolutely positioned elements are out of flow, placed in a different place in the tree, and mapped to the real frame. A placeholder frame is where they should have been.

Figure : The render tree and the corresponding DOM tree (3.1). 

The "Viewport" is the initial containing block. In Webkit it will be the "RenderView" object.

4.2The flow of constructing the tree

In Firefox, the presentation is registered as a listener for DOM updates. The presentation delegates frame creation to theFrameConstructor and the constructor resolves style (see style computation) and creates a frame.

In Webkit the process of resolving the style and creating a renderer is called "attachment". Every DOM node has an "attach" method. Attachment is synchronous, node insertion to the DOM tree calls the new node "attach" method.

Processing the html and body tags results in the construction of the render tree root. The root render object corresponds to what the CSS spec calls the containing block - the top most block that contains all other blocks. Its dimensions are the viewport - the browser window display area dimensions. Firefox calls it ViewPortFrame and Webkit calls it RenderView. This is the render object that the document point to. The rest of the tree is constructed as a DOM nodes insertion.

See the CSS2 spec on the processing model.

4.3Style Computation

Building the render tree requires calculating the visual properties of each render object. This is done by calculating the style properties of each element.

The style includes style sheets of various origins, inline style elements and visual properties in the HTML (like the "bgcolor" property).The later is translated to matching CSS style properties.

The origins of style sheets are the browser's default style sheets, the style sheets provided by the page author and user style sheets - these are style sheets provides by the browser user (browsers let you define your favorite style. In Firefox, for instance, this is done by placing a style sheet in the "Firefox Profile" folder).

Style computation brings up a few difficulties:

1. Style data is a very large construct, holding the numerous style properties, this can cause memory problems.

2. Finding the matching rules for each element can cause performance issues if it's not optimized. Traversing the entire rule list for each element to find matches is a heavy task. Selectors can have complex structure that can cause the matching process to start on a seemingly promising path that is proven to be futile and another path has to be tried.

   For example - this compound selector:

   div div div div{...} Means the rules apply to a <div> who is the descendant of 3 divs. Suppose you want to check if the rule applies for a given <div> element. You choose a certain path up the tree for checking. You may need to traverse the node tree up just to find out there are only two divs and the rule does not apply. You then need to try other paths in the tree.
3. Applying the rules involves quite complex cascade rules that define the hierarchy of the rules.

Let's see how the browsers face these issues:

4.3.1Sharing style data

Webkit nodes references style objects (RenderStyle) These objects can be shared by nodes in some conditions. The nodes are siblings or cousins and:

1. The elements must be in the same mouse state (e.g., one can't be in :hover while the other isn't)
2. Neither element should have an id
3. The tag names should match
4. The class attributes should match
5. The set of mapped attributes must be identical
6. The link states must match
7. The focus states must match
8. Neither element should be affected by attribute selectors, where affected is defined as having any selector match that uses an attribute selector in any position within the selector at all
9. There must be no inline style attribute on the elements
10. There must be no sibling selectors in use at all. WebCore simply throws a global switch when any sibling selector is encountered and disables style sharing for the entire document when they are present. This includes the + selector and selectors like :first-child and :last-child.

4.3.2Firefox rule tree

Firefox has two extra trees for easier style computation - the rule tree and style context tree. Webkit also has style objects but they are not stored in a tree like the style context tree, only the DOM node points to its relevant style.

Figure : Firefox style context tree(2.2)

The style contexts contain end values. The values are computed by applying all the matching rules in the correct order and performing manipulations that transform them from logical to concrete values. For example - if the logical value is percentage of the screen it will be calculated and transformed to absolute units. The rule tree idea is really clever. It enables sharing these values between nodes to avoid computing them again. This also saves space.

All the matched rules are stored in a tree. The bottom nodes in a path have higher priority. The tree contains all the paths for rule matches that were found. Storing the rules is done lazily. The tree isn't calculated at the beginning for every node, but whenever a node style needs to be computed the computed paths are added to the tree.

The idea is to see the tree paths as words in a lexicon. Lets say we already computed this rule tree:

Suppose we need to match rules for another element in the content tree, and find out the matched rules (in the correct order) are B - E - I. We already have this path in the tree because we already computed path A - B - E - I - L. We will now have less work to do.

 

Let's see how the tree saves us work.

Division into structs

The style contexts are divided into structs. Those structs contain style information for a certain category like border or color. All the properties in a struct are either inherited or non inherited. Inherited properties are properties that unless defined by the element, are inherited from its parent. Non inherited properties (called "reset" properties) use default values if not defined.

The tree helps us by caching entire structs (containing the computed end values) in the tree. The idea is that if the bottom node didn't supply a definition for a struct, a cached struct in an upper node can be used.

Computing the style contexts using the rule tree

When computing the style context for a certain element, we first compute a path in the rule tree or use an existing one. We then begin to apply the rules in the path to fill the structs in our new style context. We start at the bottom node of the path - the one with the highest precedence (usually the most specific selector) and traverse the tree up until our struct is full. If there is no specification for the struct in that rule node, then we can greatly optimize - we go up the tree until we find a node that specifies it fully and simply point to it - that's the best optimization - the entire struct is shared. This saves computation of end values and memory. 
If we find partial definitions we go up the tree until the struct is filled.

If we didn't find any definitions for our struct, then in case the struct is an "inherited" type - we point to the struct of our parent in the context tree, in this case we also succeeded in sharing structs. If its a reset struct then default values will be used.

If the most specific node does add values then we need to do some extra calculations for transforming it to actual values. We then cache the result in the tree node so it can be used by children.

In case an element has a sibling or a brother that points to the same tree node then the entire style context can be shared between them.

Lets see an example: Suppose we have this HTML

<html><body><divclass="err"id="div1"><p> this is a <spanclass="big"> big error </span> this is also a <spanclass="big"> very big error</span> error </p></div><divclass="err"id="div2">another error</div></body></html> And the following rules:

1. div {margin:5px;color:black}
2. .err {color:red}
3. .big {margin-top:3px}
4. div span {margin-bottom:4px}
5. #div1 {color:blue}
6. #div 2{color:green}

To simplify things let's say we need to fill out only two structs - the color struct and the margin struct. The color struct contains only one member - the color The margin struct contains the four sides. 
The resulting rule tree will look like this (the nodes are marked with the node name : the # of rule they point at):

Figure : The rule tree
The context tree will look like this (node name : rule node they point to):

Figure : The context tree

Suppose we parse the HTML and get to the second <div> tag. We need to create a style context for this node and fill its style structs. 
We will match the rules and discover that the matching rules for the <div> are 1 ,2 and 6. This means there is already an existing path in the tree that our element can use and we just need to add another node to it for rule 6 (node F in the rule tree). 
We will create a style context and put it in the context tree. The new style context will point to node F in the rule tree.

We now need to fill the style structs. We will begin by filling out the margin struct. Since the last rule node(F) doesn't add to the margin struct, we can go up the tree until we find a cached struct computed in a previous node insertion and use it. We will find it on node B, which is the uppermost node that specified margin rules.

We do have a definition for the color struct, so we can't use a cached struct. Since color has one attribute we don't need to go up the tree to fill other attributes. We will compute the end value (convert string to RGB etc) and cache the computed struct on this node.

The work on the second <span> element is even easier. We will match the rules and come to the conclusion that it points to rule G, like the previous span. Since we have siblings that point to the same node, we can share the entire style context and just point to the context of the previous span.

For structs that contain rules that are inherited from the parent, caching is done on the context tree (the color property is actually inherited, but Firefox treats it as reset and caches it on the rule tree). 
For instance if we added rules for fonts in a paragraph:

p {font-family:Verdana;font size:10px;font-weight:bold} Then the div element, which is a child of the paragraph in the context tree, could have shared the same font struct as his parent. This is if no font rules where specified for the "div".

 

In Webkit, who does not have a rule tree, the matched declarations are traversed 4 times. First non important high priority properties (properties that should be applied first because others depend on them - like display) are applied, than high priority important, then normal priority non important, then normal priority important rules. This means that properties that appear multiple times will be resolved according to the correct cascade order. The last wins. 

So to summarize - sharing the style objects (entirely or some of the structs inside them) solves issues 1 and 3. Firefox rule tree also helps in applying the properties in the correct order.

4.3.3Manipulating the rules for an easy match

There are several sources for style rules:

• CSS rules, either in external style sheets or in style elements. p {color:blue}
• Inline style attributes like <pstyle="color:blue"/>
• HTML visual attributes (which are mapped to relevant style rules) <pbgcolor="blue"/>

The last two are easily matched to the element since he owns the style attributes and HTML attributes can be mapped using the element as the key.

As noted previously in issue #2, the CSS rule matching can be trickier. To solve the difficulty, the rules are manipulated for easier access.

After parsing the style sheet, the rules are added one of several hash maps, according to the selector. There are maps by id, by class name, by tag name and a general map for anything that doesn't fit into those categories. If the selector is an id, the rule will be added to the id map, if it's a class it will be added to the class map etc. 
This manipulation makes it much easier to match rules. There is no need to look in every declaration - we can extract the relevant rules for an element from the maps. This optimization eliminates 95+% of the rules, so that they need not even be considered during the matching process(4.1).

Let's see for example the following style rules:

p.error {color:red}#messageDiv {height:50px} div {margin:5px} The first rule will be inserted into the class map. The second into the id map and the third into the tag map. 
For the following HTML fragment; <pclass="error">an error occurred </p><divid=" messageDiv">this is a message</div>

 

We will first try to find rules for the p element. The class map will contain an "error" key under which the rule for "p.error" is found. The div element will have relevant rules in the id map (the key is the id) and the tag map. So the only work left is finding out which of the rules that were extracted by the keys really match. 
For example if the rule for the div was

table div {margin:5px} it will still be extracted from the tag map, because the key is the rightmost selector, but it would not match our div element, who does not have a table ancestor.

 

Both Webkit and Firefox do this manipulation.

4.3.4Applying the rules in the correct cascade order

The style object has properties corresponding to every visual attribute (all css attributes but more generic). If the property is not defined by any of the matched rules - then some properties can be inherited by the parent element style object. Other properties have default values.

The problem begins when there is more than one definition - here comes the cascade order to solve the issue.

Style sheet cascade order

A declaration for a style property can appear in several style sheets, and several times inside a style sheet. This means the order of applying the rules is very important. This is called the "cascade" order. According to CSS2 spec, the cascade order is (from low to high):

1. Browser declarations
2. User normal declarations
3. Author normal declarations
4. Author important declarations
5. User important declarations

 

The browser declarations are least important and the user overrides the author only if the declaration was marked as important. Declarations with the same order will be sorted by specificity and then the order they are specified. The HTML visual attributes are translated to matching CSS declarations . They are treated as author rules with low priority.

Specificity

The selector specificity is defined by the CSS2 specification as follows:

• count 1 if the declaration is from is a 'style' attribute rather than a rule with a selector, 0 otherwise (= a)
• count the number of ID attributes in the selector (= b)
• count the number of other attributes and pseudo-classes in the selector (= c)
• count the number of element names and pseudo-elements in the selector (= d)

Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the specificity.

 

The number base you need to use is defined by the highest count you have in one of the categories. 
For example, if a=14 you can use hexadecimal base. In the unlikely case where a=17 you will need a 17 digits number base. The later situation can happen with a selector like this: html body div div p ... (17 tags in your selector.. not very likely).

Some examples:

*{}/* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */ li {}/* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */ li:first-line {}/* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */ ul li {}/* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */ ul ol+li {}/* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */ h1 +*[rel=up]{}/* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */ ul ol li.red {}/* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */ li.red.level {}/* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */#x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */ style=""/* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */

 

Sorting the rules

After the rules are matched, they are sorted according to the cascade rules. Webkit uses bubble sort for small lists and merge sort for big ones. Webkit implements sorting by overriding the ">" operator for the rules:

staticbooloperator>(CSSRuleData& r1,CSSRuleData& r2){int spec1 = r1.selector()->specificity();int spec2 = r2.selector()->specificity();return(spec1 == spec2): r1.position()> r2.position(): spec1 > spec2;}

 

4.4Gradual process

Webkit uses a flag that marks if all top level style sheets (including @imports) have been loaded. If the style is not fully loaded when attaching - place holders are used and it s marked in the document, and they will be recalculated once the style sheets were loaded.

Chapter 5

Layout

When the renderer is created and added to the tree, it does not have a position and size. Calculating these values is called layout or reflow.

HTML uses a flow based layout model, meaning that most of the time it is possible to compute the geometry in a single pass. Elements later ``in the flow'' typically do not affect the geometry of elements that are earlier ``in the flow'', so layout can proceed left-to-right, top-to-bottom through the document. There are exceptions - for example, HTML tables may require more than one pass (3.5).

The coordinate system is relative to the root frame. Top and left coordinates are used.

Layout is a recursive process. It begins at the root renderer, which corresponds to the <html> element of the HTML document. Layout continues recursively through some or all of the frame hierarchy, computing geometric information for each renderer that requires it.

The position of the root renderer is 0,0 and its dimensions is the viewport - the visible part of the browser window.

All renderers have a "layout" or "reflow" method, each renderer invokes the layout method of its children that need layout.

5.1Dirty bit system

In order not to do a full layout for every small change, browser use a "dirty bit" system. A renderer that is changed or added marks itself and its children as "dirty" - needing layout.

There are two flags - "dirty" and "children are dirty". Children are dirty means that although the renderer itself may be ok, it has at least one child that needs a layout.

5.2Global and incremental layout

Layout can be triggered on the entire render tree - this is "global" layout. This can happen as a result of:

1. A global style change that affects all renderers, like a font size change.
2. As a result of a screen being resized

 

Layout can be incremental, only the dirty renderers will be layed out (this can cause some damage which will require extra layouts). 
Incremental layout is triggered (asynchronously) when renderers are dirty. For example when new renderers are appended to the render tree after extra content came from the network and was added to the DOM tree.

Figure : Incremental layout - only dirty renderers and their children are layed out (3.6).

5.3Asynchronous and Synchronous layout

Incremental layout is done asynchronously. Firefox queues "reflow commands" for incremental layouts and a scheduler triggers batch execution of these commands. Webkit also has a timer that executes an incremental layout - the tree is traversed and "dirty" renderers are layout out. 
Scripts asking for style information, like "offsightHeight" can trigger incremental layout synchronously. 
Global layout will usually be triggered synchronously. 
Sometimes layout is triggered as a callback after an initial layout because some attributes , like the scrolling position changed.

5.4Optimizations

When a layout is triggered by a "resize" or a change in the renderer position(and not size), the renders sizes are taken from a cache and not recalculated.. 
In some cases - only a sub tree is modified and layout does not start from the root. This can happen in cases where the change is local and does not affect its surroundings - like text inserted into text fields (otherwise every keystroke would have triggered a layout starting from the root).

 

5.5The layout process

The layout usually has the following pattern:

1. Parent renderer determines its own width.
2. Parent goes over children and:
   1. Place the child renderer (sets its x and y).
   2. Calls child layout if needed(they are dirty or we are in a global layout or some other reason) - this calculates the child's height.
3. Parent uses children accumulative heights and the heights of the margins and paddings to set it own height - this will be used by the parent renderer's parent.
4. Sets its dirty bit to false.

 

Firefox uses a "state" object(nsHTMLReflowState) as a parameter to layout (termed "reflow"). Among others the state includes the parents width. 
The output of Firefox layout is a "metrics" object(nsHTMLReflowMetrics). It will contain the renderer computed height.

5.6Width calculation

The renderer's width is calculated using the container block's width , the renderer's style "width" property, the margins and borders. 
For example the width of the following div:

<divstyle="width:30%"/> Would be calculated by Webkit as following(class RenderBox method calcWidth):

• The container width is the maximum of the containers availableWidth and 0. The availableWidth in this case is the contentWidth which is calculated as: clientWidth()- paddingLeft()- paddingRight() clientWidth and clientHeight represent the interior of an object excluding border and scrollbar.
• The elements width is the "width" style attribute. It will be calculated as an absolute value by computing the percentage of the container width.
• The horizontal borders and paddings are now added.

So far this was the calculation of the "preferred width". Now the minimum and maximum widths will be calculated. 
If the preferred width is higher then the maximum width - the maximum width is used. If it is lower then the minimum width (the smallest unbreakable unit) hen the minimum width is used.

 

The values are cached, in case a layout is needed but the width does not change.

 

5.7Line Breaking

When a renderer in the middle of layout decides it needs to break. It stops and propagates to its parent it needs to be broken. The parent will create the extra renderers and calls layout on them.

Chapter 6

Painting

In the painting stage, the render tree is traversed and the renderers "paint" method is called to display their content on the screen. Painting uses the UI infrastructure component.

6.1Global and Incremental

Like layout, painting can also be global - the entire tree is painted - or incremental. In incremental painting, some of the renderers change in a way that does not affect the entire tree. The changed renderer invalidates it's rectangle on the screen. This causes the OS to see it as a "dirty region" and generate a "paint" event. The OS does it cleverly and coalesces several regions into one. In Chrome it is more complicated because the renderer is in a different process then the main process. Chrome simulates the OS behavior to some extent. The presentation listens to these events and delegates the message to the render root. The tree is traversed until the relevant renderer is reached. It will repaint itself (and usually its children).

6.2The painting order

CSS2 defines the order of the painting process. This is actually the order in which the elements are stacked in thestacking contexts. This order affects painting since the stacks are painted from back to front. The stacking order of a block renderer is:

1. background color
2. background image
3. border
4. children
5. outline

 

6.3Firefox display list

Firefox goes over the render tree and builds a display list for the painted rectangular. It contains the renderers relevant for the rectangular, in the right painting order (backgrounds of the renderers, then borders etc). That way the tree needs to be traversed only once for a repaint instead of several times - painting all backgrounds, then all images , then all borders etc.

Firefox optimizes the process by not adding elements that will be hidden, like elements completely beneath other opaque elements.

6.4Webkit rectangle storage

Before repainting, webkit saves the old rectangle as a bitmap. It then paints only the delta between the new and old rectangles. 

Chapter 7

Dynamic changes

The browsers try to do the minimal possible actions in response to a change. So changes to an elements color will cause only repaint of the element. Changes to the element position will cause layout and repaint of the element, its children and possibly siblings. Adding a DOM node will cause layout and repaint of the node. Major changes, like increasing font size of the "html" element, will cause invalidation of caches, relyout and repaint of the entire tree.

Chapter 8

The rendering engine's threads

The rendering engine is single threaded. Almost everything, except network operations, happens in a single thread. In Firefox and safari this is the main thread of the browser. In chrome it's the tab process main thread. 
Network operations can be performed by several parallel threads. The number of parallel connections is limited (usually 2 - 6 connections. Firefox 3, for example, uses 6).

8.1Event loop

The browser main thread is an event loop. Its an infinite loop that keeps the process alive. It waits for events (like layout and paint events) and processes them. This is Firefox code for the main event loop: while(!mExiting) NS_ProcessNextEvent(thread);

Chapter 9

CSS2 visual model

9.1The canvas

According to CSS2 specification, the term canvas describes "the space where the formatting structure is rendered." - where the browser paints the content. The canvas is infinite for each dimension of the space but browsers choose an initial width based on the dimensions of the viewport.

According to www.w3.org/TR/CSS2/zindex.html, the canvas is transparent if contained within another, and given a browser defined color if it is not.

9.2CSS Box model

The CSS box model describes the rectangular boxes that are generated for elements in the document tree and laid out according to the visual formatting model. 
Each box has a content area (e.g., text, an image, etc.) and optional surrounding padding, border, and margin areas.

Figure : CSS2 box model

Each node generates 0..n such boxes. 

All elements have a "display" property that determines their type of box that will be generated. Examples:

block - generates a block box.inline- generates one or more inline boxes. none -no box is generated. The default is inline but the browser style sheet set other defaults. For example - the default display for "div" element is block. 
You can find a default style sheet example here: www.w3.org/TR/CSS2/sample.html

 

9.3Positioning scheme

There are three schemes:

1. Normal - the object is positioned according to its place in the document - this means its place in the render tree is like its place in the dom tree and layed out according to its box type and dimensions
2. Float - the object is first layed out like normal flow, then moved as far left or right as possible
3. Absolute - the object is put in the render tree differently than its place in the DOM tree

 

The positioning scheme is set by the "position" property and the "float" attribute.

• static and relative cause a normal flow
• absolute and fixed cause an absolute positioning

In static positioning no position is defined and the default positioning is used. In the other schemes, the author specifies the position - top,bottom,left,right.

 

The way the box is layed out is determined by:

• Box type
• Box dimensions
• Positioning scheme
• External information - like images size and the size of the screen

 

9.4Box types

Block box: forms a block - have their own rectangle on the browser window.

Figure : Block box

Inline box: does not have its own block, but is inside a containing block.

Figure : Inline boxes

Blocks are formatted vertically one after the other. Inlines are formatted horizontally.

Figure : Block and Inline formatting

Inline boxes are put inside lines or "line boxes". The lines are at least as tall as the tallest box but can be taller, when the boxes are aligned "baseline" - meaning the bottom part of an element is aligned at a point of another box other then the bottom. In case the container width is not enough, the inlines will be put in several lines. This is usually what happens in a paragraph.

Figure : Lines

9.5Positioning

9.5.1Relative

Relative positioning - positioned like usual and then moved by the required delta.

Figure : Relative positioning

9.5.2Floats

A float box is shifted to the left or right of a line. The interesting feature is that the other boxes flow around it The HTML:

<p><imgstyle="float:right"src="images/image.gif"width="100"height="100"> Lorem ipsum dolor sit amet, consectetuer... </p> Will look like:

Figure : Float

9.5.3Absolute and fixed

The layout is defined exactly regardless of the normal flow. The element does not participate in the normal flow. The dimensions are relative to the container. In fixed - the container is the view port.

Figure : Fixed positioning
Note - the fixed box will not move even when the document is scrolled!

 

9.6Layered representation

It is specified by the z-index CSS property. It represents the 3rd dimension of the box, its position along the "z axis".

The boxes are divided to stacks (called stacking contexts). In each stack the back elements will be painted first and the forward elements on top, closer to the user. In case of overlap the will hide the former element. 
The stacks are ordered according to the z-index property. Boxes with "z-index" property form a local stack. The viewport has the outer stack.

Example:

<styletype="text/css"> div {position: absolute;left:2in;top:2in;}</style><p><divstyle="z-index:3;background-color:red;width:1in;height:1in;"></div><divstyle="z-index:1;background-color:green;width:2in;height:2in;"></div></p> The result will be this:

Figure : Fixed positioning

Although the red div precedes the green one in the markup, and would have been painted before in the regular flow, the z-index property is higher, so it is more forward in the stack held by the root box.

Chapter 10

Resources

1. Browser architecture
   1. Grosskurth, Alan. A Reference Architecture for Web Browsers (pdf)
   2. Gupta, Vineet. How Browsers Work - Part 1 - Architecture
2. Parsing
   1. Aho, Sethi, Ullman, Compilers: Principles, Techniques, and Tools (aka the "Dragon book"), Addison-Wesley, 1986
   2. Rick Jelliffe. The Bold and the Beautiful: two new drafts for HTML 5.
3. Firefox
   1. L. David Baron, Faster HTML and CSS: Layout Engine Internals for Web Developers.
   2. L. David Baron, Faster HTML and CSS: Layout Engine Internals for Web Developers (Google tech talk video)
   3. L. David Baron, Mozilla's Layout Engine
   4. L. David Baron, Mozilla Style System Documentation
   5. Chris Waterson, Notes on HTML Reflow
   6. Chris Waterson, Gecko Overview
   7. Alexander Larsson, The life of an HTML HTTP request
4. Webkit
   1. David Hyatt, Implementing CSS(part 1)
   2. David Hyatt, An Overview of WebCore
   3. David Hyatt, WebCore Rendering
   4. David Hyatt, The FOUC Problem
5. W3C Specifications
   1. HTML 4.01 Specification
   2. W3C HTML5 Specification
   3. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification
6. Browsers build instructions
   1. Firefox. https://developer.mozilla.org/en/Build_Documentation
   2. Webkit. http://webkit.org/building/build.html

Tali Garsiel is a developer in Israel. She started as a web developer in 2000, and became aquainted with Netscape's "evil" layer model. Just like Richard Feynmann, she had a fascination for figuring out how things work so she began digging into browser internals and documenting what she found. Tali also has published a short guide on client-side performance.