So far, layout has been a linear process that handles open tags and 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_x and 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