So far, layout has been a linear process that handles open tags and close tags independently. But web pages are trees, and look like them: borders and backgrounds visually nest inside one another. To support that, this chapter switches to tree-based layout, where the tree of elements is transformed into a tree of layout objects for the visual elements of the page. In the process, we’ll make our web pages more colorful with backgrounds.
Right now, our browser lays out an element’s open and close tags separately. Both tags modify global state, like the
cursor_y variables, but they aren’t otherwise connected, and information about the element as a whole, like its width and height, is never computed. That makes it pretty hard to draw a background color behind text. So web browsers structure layout differently.
In a browser, layout is about producing a layout tree, whose nodes are layout objects, each associated with an HTML element,Elements like
<script> don’t generate layout objects, and some elements generate multiple (
<li> elements have a layout object for the bullet point!), but mostly it’s one layout object each. and each with a size and a position. The browser walks the HTML tree to produce the layout tree, then computes the size and position for each layout object, and finally draws each layout object to the screen.
Let’s start a new class called
BlockLayout, which will represent a node in the layout tree. Like our
Element class, layout objects form a tree, so they have a list of
children and a
parent. We’ll also have a
node field for the HTML element the layout object corresponds to.
class BlockLayout: def __init__(self, node, parent, previous): self.node = node self.parent = parent self.previous = previous self.children = 
I’ve also added a field for the layout object’s previous sibling. We’ll need it to compute sizes and positions.
Each layout object also needs a size and position, which we’ll store in the
y fields. But let’s leave that for later. The first job for
BlockLayout is creating the layout tree itself.
We’ll do that in a new
layout method, looping over each child node and creating a new child layout object for it.
class BlockLayout: def layout(self): = None previous for child in self.node.children: next = BlockLayout(child, self, previous) self.children.append(next) = next previous
This code is tricky because it involves two trees. The
child are part of the HTML tree; but
next are part of the layout tree. The two trees have similar structure, so it’s easy to get confused. But remember that this code constructs the layout tree from the HTML tree, so it reads from
node.children (in the HTML tree) and writes to
self.children (in the layout tree).
Anyway, this code creates layout objects for the direct children of the node in question. Now those children’s own
layout methods can be called to build the whole tree recursively:
def layout(self): # ... for child in self.children: child.layout()
We’ll discuss the base case of the recursion in just a moment, but first let’s ask how it starts. Inconveniently, the
BlockLayout constructor requires a parent node, so we need another kind of layout object at the root.You couldn’t just use
None for the parent, because the root layout object also computes its size and position differently, as we’ll see later this chapter. I think of that root as the document itself, so let’s call it
class DocumentLayout: def __init__(self, node): self.node = node self.parent = None self.children =  def layout(self): = BlockLayout(self.node, self, None) child self.children.append(child) child.layout()
So we’re building a layout tree with one layout object per HTML node, plus an extra layout object at the root, by recursively calling
layout. It looks like this:
In this example there are four
BlockLayout objects, in green, one per element. There’s also a
DocumentLayout at the root.
The browser must now move on to computing sizes and positions for each layout object. But before we write that code, we have to face an important truth: different HTML elements are laid out differently. They need different kinds of layout objects!
The layout tree isn’t accessible to web developers, so it hasn’t been standardized, and its structure differs between browsers. Even the names don’t match! Chrome calls it a layout tree, Safari a render tree, and Firefox a frame tree.
<header> contain blocks stacked vertically. But elements like
<h1> contain text and lay that text out horizontally in lines.In European languages, at least! Abstracting a bit, there are two layout modes, two ways an element can be laid out relative to its children: block layout and inline layout.
We’ve already got
BlockLayout for block layout. And actually, we’ve already got inline layout too: it’s just the text layout we’ve been implementing since Chapter 2. So let’s rename the existing
Layout class to
InlineLayout and refactor to match methods with
InlineLayout and rename its constructor to
layout. Add a new constructor similar to
class InlineLayout: def __init__(self, node, parent, previous): self.node = node self.parent = parent self.previous = previous self.children = 
In the new
layout method, replace the
tree argument with the
class InlineLayout: def layout(self): # ... self.line =  self.recurse(self.node) self.flush()
Let’s also initialize
y instead of
VSTEP, both in
class InlineLayout: def layout(self): # ... self.cursor_x = self.x self.cursor_y = self.y # ... def flush(self): # ... self.cursor_x = self.x # ...
Inline layout objects aren’t going to have any children
layout. So the new
InlineLayout now matches
BlockLayout’s methods. Just as with block layout, let’s leave actually computing
height to later.
Our tree-creation code now needs to use the right layout object for each element. Normally this is easy: things with text in them get
InlineLayout, things with block elements like
<div> inside get
BlockLayout. But what happens if an element contains both? In some sense, this is an error on the part of the web developer. And just like with implicit tags in Chapter 4, browsers use a repair mechanism to make sense of the situation. In real browsers, “anonymous block boxes” are used, but in our toy browser we’ll implement something a little simpler.
Here’s a list of block elements:Taken from the HTML5 living standard.
= [ BLOCK_ELEMENTS "html", "body", "article", "section", "nav", "aside", "h1", "h2", "h3", "h4", "h5", "h6", "hgroup", "header", "footer", "address", "p", "hr", "ol", "ul", "menu", "li", "dl", "dt", "dd", "figure", "figcaption", "main", "div", "table", "form", "fieldset", "legend", "details", "summary", ]
BlockLayout for elements with children in that list, and
InlineLayout otherwise. Put that logic in a new
def layout_mode(node): if isinstance(node, Text): return "inline" elif node.children: for child in node.children: if isinstance(child, Text): continue if child.tag in BLOCK_ELEMENTS: return "block" return "inline" else: return "block"
This function additionally makes sure text nodes get inline layout while empty elements get block layout. Now we can call
layout_mode to determine which layout mode to use for each element:
class BlockLayout: def layout(self): = None previous for child in self.node.children: if layout_mode(child) == "inline": next = InlineLayout(child, self, previous) else: next = BlockLayout(child, self, previous) self.children.append(next) = next previous # ...
Our layout tree now has a
DocumentLayout at the root,
BlockLayouts at interior nodes, and
InlineLayouts at the leaves:Or, the leaf nodes could be
BlockLayouts for empty elements.
With the layout tree built, we can finally move on to computing the sizes and positions for the layout objects in the tree.
In CSS, the layout mode is set by the
display property. The oldest CSS layout modes, like
block, are set on the children instead of the parent, which leads to hiccups like anonymous block boxes. Newer properties like
grid are set on the parent. This chapter uses the newer, less confusing convention, even though it’s actually implementing inline and block layout.
By default, layout objects are greedy and take up all the horizontal space they can.In the next chapter, we’ll add support for user styles, which modify these rules and allow setting custom widths, borders, or padding. So their width is their parent’s width:
self.width = self.parent.width
And each layout object starts at its parent’s left edge:
self.x = self.parent.x
The vertical position of a layout object depends on the position and height of their previous sibling. If there is no previous sibling, they start at the parent’s top edge:
if self.previous: self.y = self.previous.y + self.previous.height else: self.y = self.parent.y
These three computations have to go before the recursive call to each child’s
layout method. After all, a layout object’s width depends on the parent’s width; so the width must be computed before laying out the children. The position is the same: it depends on both the parent and previous sibling, so the parent has to compute it before recursing, and when recursing it has to lay out the children in order.
Height is the opposite. A
BlockLayout should be tall enough to contain all of its children, so its height should be the sum of its children’s heights:
self.height = sum([child.height for child in self.children])
BlockLayout’s height depends on the height of its children, its height must be computed after recursing to compute the heights of its children. Getting this dependency order right is crucial: get it wrong, and some layout object will try to read a value that hasn’t been computed yet, and the browser will have a bug.
y the same way, but
height is a little different: an
InlineLayout has to contain all of the text inside it, which means its height must be computed from its y-cursor.
class InlineLayout: def layout(self): # ... self.height = self.cursor_y - self.y
y have to be computed before text is laid out, but
height has to be computed after. It’s all about that dependency order.
DocumentLayout needs some layout code, though since the document always starts in the same place it’s pretty simple:
class DocumentLayout: def layout(self): # ... self.width = WIDTH - 2*HSTEP self.x = HSTEP self.y = VSTEP child.layout()self.height = child.height + 2*VSTEP
Note that there’s some padding around the contents—
HSTEP on the left and right, and
VSTEP above and below. That’s so the text won’t run into the very edge of the window and get cut off.
For all three types of layout object, the order of the steps in the
layout method should be the same:
layoutis called, it first creates a child layout object for each child element.
yfields, reading from the
heightfield, reading from the child layout objects.
Sticking to this order is necessary to satisfy the the dependencies between size and position fields;will explore this topic in more detail.
Formally, computations on a tree like this can be described by an attribute grammar. Attribute grammar engines analyze dependencies between different attributes to determine the right order to traverse the tree and calculate each attribute.
Now that our layout objects have size and position information, our browser should use that information to render the page itself. First, we need to run layout in the browser’s
class Browser: def load(self, url): = request(url) headers, body = HTMLParser(body).parse() nodes self.document = DocumentLayout(nodes) self.document.layout()
Recall that our browser draws a web page by first collecting a display list and then calling
paint to draw the things in the list. With tree-based layout, we collect the display list by recursing down the layout tree.
I think it’s most convenient to do that by adding a
paint function to each layout object which does the recursion. A neat trick here is to pass the list itself as an argument, and have the recursive function append to that list. For
DocumentLayout, which only has one child, the recursion looks like this:
class DocumentLayout: def paint(self, display_list): self.children.paint(display_list)
BlockLayout, which has multiple children,
paint is called on each child:
class BlockLayout: def paint(self, display_list): for child in self.children: child.paint(display_list)
InlineLayout is already storing things to paint in its
display_list variable, so we can copy them over:
class InlineLayout: def paint(self, display_list): self.display_list) display_list.extend(
Now the browser can use
paint to collect its own
class Browser: def load(self, url): # ... self.display_list =  self.document.paint(self.display_list) self.draw()
Check it out: your browser is now using fancy tree-based layout! I recommend pausing to test and debug. Tree-based layout is powerful but complex, and we’re about to add more features. Stable foundations make for comfortable houses.
Layout trees are common in GUI frameworks, but there are other ways to structure layout, such as constraint-based layout. TeX’s boxes and glue and iOS auto-layout are two examples of this alternative paradigm.
Browsers use the layout tree a lot,For example, in , we’ll use the size and position of each link to figure out which one the user clicked on! and one simple and visually compelling use case is drawing backgrounds.
Backgrounds are rectangles, so our first task is putting rectangles in the display list. Conceptually, the display list contains commands, and we want two types of commands:
class DrawText: def __init__(self, x1, y1, text, font): self.top = y1 self.left = x1 self.text = text self.font = font class DrawRect: def __init__(self, x1, y1, x2, y2, color): self.top = y1 self.left = x1 self.bottom = y2 self.right = x2 self.color = color
InlineLayout must add
DrawText objects to the display list:Why not change
display_list to contain
DrawText commands directly? You could, but it would be a bit harder to refactor later.
class InlineLayout: def paint(self, display_list): for x, y, word, font in self.display_list: display_list.append(DrawText(x, y, word, font))
BlockLayout can add
DrawRect commands for backgrounds. Let’s add a gray background to
pre tags (which are used for code examples):
class BlockLayout: def paint(self, display_list): if self.node.tag == "pre": = self.x + self.width, self.y + self.height x2, y2 = DrawRect(self.x, self.y, x2, y2, "gray") rect display_list.append(rect)# ...
Make sure this code comes before the recursive
paint call on child layout objects: the background has to be drawn below and therefore before the text inside the source block.
With the display list filled out, we need the
paint method to run each graphics command. Let’s add an
execute method for this. On
DrawText it calls
class DrawText: def execute(self, scroll, canvas): canvas.create_text(self.left, self.top - scroll, =self.text, text=self.font, font='nw', anchor )
execute takes the scroll amount as a parameter; this way, each graphics command does the relevant coordinate conversion itself.
DrawRect does the same with
class DrawRect: def execute(self, scroll, canvas): canvas.create_rectangle(self.left, self.top - scroll, self.right, self.bottom - scroll, =0, width=self.color, fill )
create_rectangle draws a one-pixel black border, which for backgrounds we don’t want, so make sure to pass
width = 0:
We still want to skip offscreen graphics commands, so let’s add a
bottom field to
DrawText so we know when to skip those:
def __init__(self, x1, y1, text, font): # ... self.bottom = y1 + font.metrics("linespace")
draw method now just uses
bottom to decide which commands to
def draw(self): self.canvas.delete("all") for cmd in self.display_list: if cmd.top > self.scroll + HEIGHT: continue if cmd.bottom < self.scroll: continue self.scroll, self.canvas) cmd.execute(
Try your browser on a page—maybe this one—with code snippets on it. You should see each code snippet set off with a gray background.
On some systems, the
metrics commands are awfully slow. Adding another call makes things even slower.
metrics call duplicates a call in
flush. If you’re careful you can pass the results of that call to
DrawText as an argument.
Here’s one more cute benefit of tree-based layout. Thanks to tree-based layout we now record the height of the whole page. The browser can use that to avoid scrolling past the bottom of the page:
def scrolldown(self, e): = self.document.height - HEIGHT max_y self.scroll = min(self.scroll + SCROLL_STEP, max_y) self.draw()
So that’s the basics of tree-based layout! In fact, as we’ll see in the next two chapters, this is just part of the layout tree’s role in the browser. But before we get to that, we need to add some styling capabilities to our browser.
This chapter was a dramatic rewrite of your browser’s layout engine:
Tree-based layout makes it possible to dramatically expand our browser’s styling capabilities. We’ll work on that in the next chapter.
The complete set of functions, classes, and methods in our browser should look something like this:
def __init__(text, parent)
def __init__(tag, attributes, parent)
def print_tree(node, indent)
def __init__(node, parent, previous)
def __init__(node, parent, previous)
def __init__(x1, y1, text, font)
def execute(scroll, canvas)
def __init__(x1, y1, x2, y2, color)
def execute(scroll, canvas)
if __name__ == "__main__"
Links Bar: At the top and bottom of each chapter of this book is a gray bar naming the chapter and offering back and forward links. It is enclosed in a
<nav class="links"> tag. Have your browser give this links bar the light gray background a real browser would.
Hidden Head: There’s a good chance your browser is still showing scripts, styles, and page titles at the top of every page you visit. Make it so that the
<head> element and its contents are never displayed. Those elements should still be in the HTML tree, but not in the layout tree.
Bullets: Add bullets to list items, which in HTML are
<li> tags. You can make them little squares, located to the left of the list item itself. Also indent
<li> elements so the text inside the element is to the right of the bullet point.
Scrollbar: At the right edge of the screen, draw a blue, rectangular scrollbar. The ratio of its height to the screen height should be the same as the ratio of the screen height to the document height, and its location should reflect the position of the screen within the document. Hide the scrollbar if the whole document fits onscreen.
Table of Contents: This book has a table of contents at the top of each chapter, enclosed in a
<nav id="toc"> tag, which contains a list of links. Add the text “Table of Contents”, with a gray background, above that list. Don’t modify the lexer or parser.
Anonymous block boxes: Sometimes, an element has a mix of text-like and container-like children. For example, in this HTML,
<div><i>Hello, </i><b>world!</b><p>So it began...</p></div>
<div> element has three children: the
<p> elements. The first two are text-like; the last is container-like. This is supposed to look like two paragraphs, one for the
<b> and the second for the
<p>. Make your browser do that. Specifically, modify
InlineLayout so it can be passed a sequence of sibling nodes, instead of a single node. Then, modify the algorithm that constructs the layout tree so that any sequence of text-like elements gets made into a single
Run-ins: A “run-in heading” is a heading that is drawn as part of the next paragraph’s text.The exercise names in this section could be considered run-in headings. But since browser support for the
display: run-in property is poor, this book actually doesn’t use it; the headings are actually embedded in the next paragraph. Modify your browser to render
<h6> elements as run-in headings. You’ll need to implement the previous exercise on anonymous block boxes, and then add a special case for
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