What determines the root element of the CSSOM (CSS Object Model)?

Question

What determines the root element of the CSSOM (CSS Object Model)?

As I dive into my research on browser rendering, I've reached the stage in the rendering process where the browser creates the CSSOM from raw CSS.

In many tutorials I've encountered, the authors confidently state that the body element is the root, and any styles applied to it will automatically cascade down to the rest of the DOM unless overridden by a different selector. This concept can be seen in examples like those found here: https://blog.logrocket.com/how-css-works-parsing-painting-css-in-the-critical-rendering-path-b3ee290762d3/

and also here: https://developers.google.com/web/fundamentals/performance/critical-rendering-path/constructing-the-object-model

Both explanations assume the body tag to be the root, even though the html tag may seem more fitting for this role. Additionally, the HTML specifications do not mandate the presence of either of these tags in the markup (unless I'm mistaken).

This piece of information holds significant importance when styling elements in the render tree. Without clarity on which element serves as the root, understanding how styles cascade becomes challenging.

My query revolves around whether browsers always consider the body element as the root, or if there exists a method to determine the root element in the browser's CSS Tree?

css dom cssom

Answer 1

Answer №1

There are multiple issues being mixed up here, making it challenging to mark this as a duplicate question. Even though each individual query has been asked and answered before, the amalgamation of them creates unique circumstances that require clarification. I hope this explanation helps!

How is the root element of the CSSOM (CSS Object Model) determined?

The root element is simply the topmost element in the document, also known as html. Let's be clear about that.

Authors sometimes assume that the body element is the root (...) For example, see this link: https://blog.logrocket.com/how-css-works-parsing-painting-css-in-the-critical-rendering-path-b3ee290762d3/

No, the example on that page uses "root" as the starting point of a specific subtree focusing on <body>, but it could easily have been any other element like <table>. The choice of <body> is incidental in this context.

Furthermore, the HTML specification doesn't explicitly require either of these tags in markup (although there may be some misunderstanding).

While the start tags for <html> and <body> are optional, their presence can be omitted from an HTML document. However, the elements themselves still exist within the document structure. You can verify this by loading a simple HTML snippet like:

<title>title</title>
<div>Hello world</div>

in a browser and examining the rendered DOM tree, where you'll observe the div enclosed within a body element even without the explicit tag.

It can be confusing that certain CSS styles applied to body are actually inherited by html instead in some cases. For instance, setting a background-color property on the body affects the entire browser window, not just the body content area.
This behavior stems from an older practice when the html element wasn't considered styleable, and only the body could have styling attributes like BGCOLOR, TEXT, LINK, etc. Though modern browsers have moved away from this limitation, they often retain this behavior for backward compatibility reasons.

Answer 2

There are multiple issues being mixed up here, making it challenging to mark this as a duplicate question. Even though each individual query has been asked and answered before, the amalgamation of them creates unique circumstances that require clarification. I hope this explanation helps!

How is the root element of the CSSOM (CSS Object Model) determined?

The root element is simply the topmost element in the document, also known as html. Let's be clear about that.

Authors sometimes assume that the body element is the root (...) For example, see this link: https://blog.logrocket.com/how-css-works-parsing-painting-css-in-the-critical-rendering-path-b3ee290762d3/

No, the example on that page uses "root" as the starting point of a specific subtree focusing on <body>, but it could easily have been any other element like <table>. The choice of <body> is incidental in this context.

Furthermore, the HTML specification doesn't explicitly require either of these tags in markup (although there may be some misunderstanding).

While the start tags for <html> and <body> are optional, their presence can be omitted from an HTML document. However, the elements themselves still exist within the document structure. You can verify this by loading a simple HTML snippet like:

<title>title</title>
<div>Hello world</div>

in a browser and examining the rendered DOM tree, where you'll observe the div enclosed within a body element even without the explicit tag.

It can be confusing that certain CSS styles applied to body are actually inherited by html instead in some cases. For instance, setting a background-color property on the body affects the entire browser window, not just the body content area.
This behavior stems from an older practice when the html element wasn't considered styleable, and only the body could have styling attributes like BGCOLOR, TEXT, LINK, etc. Though modern browsers have moved away from this limitation, they often retain this behavior for backward compatibility reasons.

What determines the root element of the CSSOM (CSS Object Model)?

Answer №1

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