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
<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 by looking how the existing
Layout class is
class Browser: def load(self, url): # ... self.display_list = Layout(self.nodes).display_list #...
Layout object is created briefly and then thrown
away. Let’s instead make it the beginning of our layout tree by storing
it in a
class Browser: def load(self, url): # ... self.document = Layout(self.nodes) self.document.layout() self.display_list = self.document.display_list #...
Note that I’ve renamed the
Layout constructor to a
layout method, so that constructing a layout object and
actually laying it out can be different steps. The constructor now just
stores the node it was passed:
class Layout: def __init__(self, node): self.node = node
So far, we still don’t have a tree—we just have a single
Layout object. To make it into a tree, we’ll need add child
and parent pointers. I’m also going to add a pointer to the previous
sibling, because that’ll be useful for computing sizes and positions
class Layout: def __init__(self, node, parent, previous): self.node = node self.parent = parent self.previous = previous self.children = 
That said, requiring a
element now makes it tricky to construct a
Layout object in
Browser, since the root of the layout tree obviously can’t
have a parent. To rectify that, let me add a second kind of layout
object to serve as the root of the layout tree.I don’t want to just pass
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): = Layout(self.node, self, None) child self.children.append(child) child.layout()self.display_list = child.display_list
Note an interesting thing about this new
its role is to create the child layout objects, and then
recursively call its children’s
This is a common pattern for constructing trees, and we’ll be seeing it
a lot throughout this book.
Now when we construct a
DocumentLayout object inside
load, we’ll be building a tree! A very short tree, more of
a stump for now, but it’s something!
By the way, since we now have
Layout so it’s less ambiguous. I like
BlockLayout as a name, because we ultimately want
Layout to represent a block of text, like a paragraph or a
class BlockLayout: # ...
Make sure to rename the
Layout constructor call in
DocumentLayout as well. Test your browser and make sure
that after all of these refactors, everything still works.
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.
So far, we’ve focused on text layout—and text is laid out horizontally in lines.In European languages, at least! But web pages are really constructed out of larger blocks, like headings, paragraphs, and menus, that are stacked vertically one after another. We need to add support for this kind of layout to our browser, and the way we’re going to do that involves expanding on the layout tree we’ve already built.
The core idea is that we’ll have a whole tree of
BlockLayout objects (with a
the root). Some will represent leaf blocks that contain text, and
they’ll lay out their contents the way we’ve already implemented. But
there will also be new, intermediate
BlockLayout children, and they will stack their children
To create these intermediate
BlockLayout children, we
can use a loop like this:
class BlockLayout: def layout_intermediate(self): = None previous for child in self.node.children: next = BlockLayout(child, self, previous) self.children.append(next) = next previous
I’ve called this method
layout_intermediate, but only so
you can add it to the code right away and then compare it with the
This code is tricky, so read it carefully. It involves two trees: the
HTML tree, which
child point to; and
the layout tree, which
next point to. 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).
So we have two ways to lay out an element: either calling
flush, or this
layout_intermediate function. To determine which one a
layout object should use, we’ll need to know what kind of content its
HTML node contains: text and text-related tags like
<b>, or blocks like
<h1>. That function looks something like this:
class BlockLayout: def layout_mode(self): if isinstance(self.node, Text): return "inline" elif self.node.children: if any([isinstance(child, Element) and \ in BLOCK_ELEMENTS child.tag for child in self.node.children]): return "block" else: return "inline" else: return "block"
Here the list of
BLOCK_ELEMENTS is basically what you
expect, a list of all the tags that describe parts of a page instead of
= [ BLOCK_ELEMENTS "html", "body", "article", "section", "nav", "aside", "h1", "h2", "h3", "h4", "h5", "h6", "hgroup", "header", "footer", "address", "p", "hr", "pre", "blockquote", "ol", "ul", "menu", "li", "dl", "dt", "dd", "figure", "figcaption", "main", "div", "table", "form", "fieldset", "legend", "details", "summary" ]
layout_mode method has to handle one tricky case,
where a node contains both block children like a
element but also text children like a text node or a
<b> element. I’ve chosen to use block mode in this
case, but it’s probably best to think of this as a kind of error on the
part of the web developer. And just like with implicit tags in Chapter 4, we use a repair mechanism to make sense
of the situation.In real
browsers, that repair mechanism is called “anonymous
block boxes” and is more complex than what’s described
BlockLayout can determine what kind of layout to
do based on the
layout_mode of its HTML node:
class BlockLayout: def layout(self): = self.layout_mode() mode if mode == "block": = None previous for child in self.node.children: next = BlockLayout(child, self, previous) self.children.append(next) = next previous else: self.cursor_x = 0 self.cursor_y = 0 self.weight = "normal" self.style = "roman" self.size = 16 self.line =  self.recurse(self.node) self.flush()
BlockLayouts can now have children, the
layout method next needs to recursively call
layout so those children can construct their children, and
so on recursively:
class BlockLayout: def layout(self): # ... for child in self.children: child.layout()
We also need to gather their
display_list fields into a
class BlockLayout: def layout(self): # ... for child in self.children: self.display_list.extend(child.display_list)
Our browser is now constructing a whole tree of
BlockLayout objects; in fact, if you add a
print_tree call to
method, you’ll see that large web pages like this chapter produce large
and complex layout trees!
Oh, you might also notice that the text on these web pages is now totally unreadable, because it’s all overlapping at the top of the page. Let’s fix that next.
In CSS, the layout mode is set by the
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.
In the previous chapter, the
Layout object was responsible for the whole web page, so it
just laid out its content starting at the top of the page. Now that we
BlockLayout objects each containing a
different paragraph of text, we’re going to need to do things a little
differently, computing a size and position for each layout object
Let’s start with
Instead of having them denote absolute positions on the page, let’s make
them relative to the
BlockLayout itself; they now need to
0 instead of
VSTEP, both in
class BlockLayout: def layout(self): else: self.cursor_x = 0 self.cursor_y = 0 def flush(self): # ... self.cursor_x = 0 # ...
Since these fields are now relative, we’ll need to add the block’s
y position in
computing the display list:
class BlockLayout: def flush(self): # ... for rel_x, word, font in self.line: = self.x + rel_x x = self.y + baseline - font.metrics("ascent") y self.display_list.append((x, y, word, font)) # ...
Similarly, to wrap lines, we can’t compare
cursor_x is a relative measure
WIDTH is an absolute measure; instead, we’ll wrap
cursor_x reaches the block’s
class BlockLayout: def word(self, word): # ... if self.cursor_x + w > self.width: # ... # ...
So now that leaves us with the problem of computing these
width fields. Let’s
BlockLayouts represent blocks of text like
paragraphs or headings, and are stacked vertically one atop another.
That means each one starts at its parent’s left edge:
class BlockLayout: def layout(self): self.x = self.parent.x # ...
Its vertical position depends on the position and height of their previous sibling. If there is no previous sibling, they start at the parent’s top edge:
class BlockLayout: def layout(self): if self.previous: self.y = self.previous.y + self.previous.height else: self.y = self.parent.y # ...
Note that in each of these cases, to compute one block’s
of its parent block must already have been computed. That means
these computations have to go before the recursive
layout call, so those children can compute their
y based on this block’s
y. Similarly, since the
y position of a
block depends on its previous sibling’s
y position, the
layout calls have to start at the first sibling
and iterate through the list forward—which is how we’ve already done it,
but which will be an important constraint in later chapters.
Now we’ll need compute widths and heights. Width is easy: blocks are as wide as their parents:In the next chapter, we’ll add support for author-defined styles, which in real browsers modify these layout rules by setting custom widths or changing how x and y position are computed.
class BlockLayout: def layout(self): self.width = self.parent.width # ...
Height, meanwhile, is a little tricky. A
that contains other blocks should be tall enough to contain all of its
children, so its height should be the sum of its children’s heights:
class BlockLayout: def layout(self): # ... if mode == "block": self.height = sum([ for child in self.children]) child.height
BlockLayout that contains text doesn’t have
children; instead, it needs to be tall enough to contain all its text,
which we can conveniently read off of
cursor_y:Since the height is just equal
cursor_y, why not rename
height instead? You could, it would work fine, but I would
rather not. As you can see from, say, the
height field is a public field, read by other layout
objects to compute their positions. As such I’d rather make sure it
always has the right value, whereas
changes as we lay out a paragraph of text and therefore sometimes has
the “wrong” value. Keeping these two fields separate avoids a whole
class of nasty bugs where the
height field is read “too
soon” and therefore gets the wrong value.
class BlockLayout: def layout(self): # ... else: self.height = self.cursor_y
Let’s think again about dependencies. Height has the opposite
dependencies compared to
height of a block depends on its
children’s heights. While
width must be computed before the recursive call,
height has to be computed after, at the very end
DocumentLayout needs some layout code,
though since the document always starts in the same place it’s pretty
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—
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 computes the
yfields, reading from the
heightfield, reading from the child layout objects.
You can see these steps in action in this widget:
This kind of dependency reasoning is crucial to layout and more broadly to any kind of computation on trees. If you get the order of operations wrong, some layout object will try to read a value that hasn’t been computed yet, and the browser will have a bug. We’ll come back to this issue of dependencies later, where it will become even more important.
Anyway, with all of the sizes and positions now computed correctly, you should see the browser now correctly display all of the text on the page.
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.
layout method is now doing quite a bit of work:
computing sizes and positions; creating child layout objects;
recursively laying out those child layout objects; and aggregating the
display lists so the text can be drawn to the screen. This is a bit
messy, so let’s take a moment to extract just one part of this, the
display list part. Along the way, we can stop copying the display list
contents over and over again as we go up the layout tree.
I think it’s most convenient to do that by adding a
paint function to each layout object, which appends any of
its own layout objects to the display list and then recursively paints
the child layouts. 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)
You can now delete the line that computes a
BlockLayout with multiple children,
paint is called on each child:
class BlockLayout: def paint(self, display_list): for child in self.children: child.paint(display_list)
Again, delete the line that computes a
display_list field by copying from child layout
Finally for a
BlockLayout object with text inside, we
need to copy over the
display_list field that it computes
class BlockLayout: 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 Chapter 7, 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
BlockLayout must add
to the display list:Why
not change the
display_list field inside an
BlockLayout to contain
directly? I suppose you could, but I think it’s cleaner this way, with
all of the draw commands created in one place.
class BlockLayout: def paint(self, display_list): for x, y, word, font in self.display_list: display_list.append(DrawText(x, y, word, font))# ...
Note that we must add the block’s
since the positions in the display list are relative to the block’s
But it can also add
DrawRect commands for backgrounds.
Let’s add a gray background to
pre tags (which are used for
class BlockLayout: def paint(self, display_list): if isinstance(self.node, Element) and 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 loop that adds
DrawText objects and before the recursion into
child layout objects: the background has to be drawn below and
therefore before any contents. This is again a kind of
dependency reasoning with tree traversals!
With the display list filled out, we need the
method to run each graphics command. Let’s add an
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
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
def __init__(self, x1, y1, text, font): # ... self.bottom = y1 + font.metrics("linespace")
draw method now just uses
bottom to decide which commands to
class Browser: 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
commands are awfully slow. Adding another call makes things even
metrics call duplicates a call in
flush. If you’re careful you can pass the results of that
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): = max(self.document.height - HEIGHT, 0) max_y self.scroll = min(self.scroll + SCROLL_STEP, max_y) self.draw()
So those are 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 get_font(size, weight, slant)
def __init__(text, parent)
def __init__(tag, attributes, parent)
def print_tree(node, indent)
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
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
elements so the text inside the element is to the right of the bullet
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
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
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
<b> and the second for
<p>. Make your browser do that. Specifically,
BlockLayout 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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