We all have used CSS to build beautiful and responsive webpages. But do you know:
How browser parses the CSS?
How CSS renders the style?
How layout of the page is decided?
In this post I will try to answer the above questions in a simple manner.
CSS is parsed on the browser in 3 steps.
- Resolve conflicting CSS declarations.
- Process final computed CSS values.
- Visual Formatting Model.
Here is a complete flow of these 3 steps.
Let's understand each step.
1 Resolve Conflicting CSS declarations.
- Different stylesheets are combined together.
- Resolving conflicts when more than one rule is applied to a certain element.
For ex: Which color to apply when defined twice for the same element.
Here Cascade & Specificity is at play.
2 Processing Values
All relative units (%, em, rem, vh, vw) are finally converted to absolute unit, pixels (px).
At this phase, all CSS values become computed values not the specified values.
3 The visual formatting model.
This is the algorithm that calculates boxes & determines the layout of these boxes, for each element in the render tree, to determine the page's final layout.
So, that's how CSS is parsed on the browser.
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