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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesCSS (Cascading Style Sheets) controls how web documents look. HTML provides a page’s content and structure; CSS selects elements and sets presentation details such as color, type, spacing, and layout. A browser combines CSS rules with its built-in defaults to decide how each element appears.
What is CSS?
CSS is a stylesheet language for describing how documents are presented. It is commonly used with HTML, and can also style XML documents. Its rules can describe presentation for screens, printed pages, speech, and other media. The W3C describes CSS as “a core language of the open web platform” used to add styles such as fonts, colors, and spacing to web documents. W3C: Cascading Style Sheets
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HTML and CSS have different jobs. HTML represents content and structure—for example, a heading, a paragraph, or a link. CSS describes how those elements should be presented. A CSS rule can make a heading larger, add space around a paragraph, or arrange elements in a layout without changing what the content is.
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A browser reads a document and its applicable stylesheets, works out which rules apply to each element, and computes a value for each CSS property. It then renders the result for the viewing environment. What you see reflects more than one rule: browser defaults, author styles, inheritance, property values and units, and the display environment all contribute. MDN: What is CSS?
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Recognize a CSS rule
A basic rule has a selector followed by a declaration block. The selector identifies elements the rule can match; each declaration pairs a property with a value.
p {
color: navy;
}Here, p is the selector, and color: navy; is a declaration. It sets the text color of matching paragraph elements to navy. The braces enclose the declaration block. MDN: CSS syntax
Browsers have default styles
A page can be readable even before its author adds CSS because browsers provide user-agent stylesheets: built-in rules that supply defaults for common elements. An author stylesheet can change those defaults. Seeing a heading in a larger font or paragraphs separated by space does not necessarily mean the page author wrote a rule for each effect; some presentation may come from the browser. MDN: What is CSS?
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How does the CSS cascade choose a value?
If multiple applicable declarations set the same property, the cascade determines which value is used. The name “cascading” refers to how CSS resolves these competing rules. It is not simply a contest in which the most specific-looking selector always wins: the browser considers the cascade’s precedence rules before comparing specificity. MDN: Introduction to the CSS cascade
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- Check applicability. A rule must apply to the element and property in question.
- Compare origin and importance. The cascade accounts for where a declaration comes from and whether it is marked important.
- Compare specificity. Specificity helps resolve declarations that remain in the relevant precedence context; it does not override the earlier cascade decisions.
- Consider scoping proximity where relevant. For scoped rules, how close a scope is can affect which declaration wins.
- Use source order to break a remaining tie. When declarations are otherwise tied, the one later in the relevant order can win.
This is a practical outline, not every detail of the cascade. The full rules account for additional cases and precedence levels. MDN: Introduction to the CSS cascade
What is inheritance in CSS?
Some properties can take their value from an element’s parent when no value is set directly on the element. This is inheritance. Other properties do not inherit and instead use their initial value unless another applicable rule sets them. A declaration that directly targets an element takes precedence over a value that element would otherwise inherit from an ancestor. MDN: CSS cascading and inheritance
Inheritance helps explain why one rule can affect several nested elements, but it does not mean every property automatically flows down the document tree. Whether a property inherits depends on that property’s CSS behavior. MDN: Styling the content
How to debug a CSS rule that does not seem to work
- Check the selector. Confirm that it matches the element you intended to style.
- Check the declaration. Confirm that the property and value are valid and written correctly.
- Look for competing declarations. Identify other rules that apply to the same element and property, including browser defaults and author styles.
- Compare precedence, then specificity and order. Establish which declarations win under the cascade before assuming that a selector needs to be made more specific.
- Check inheritance. If the element has no direct value, determine whether the property inherits from its parent or uses its initial value.
MDN’s guides to CSS syntax and the cascade explain the rules behind these checks.
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Is there a CSS3 or CSS4 version?
CSS should not be treated as a single sequence of unified releases where all features arrive together in “CSS3” or “CSS4.” CSS specifications develop as modules, and different modules can have their own versions and publication status. The W3C’s CSS overview identifies CSS Snapshot 2025 as listing portions considered ready for implementers. For any particular property or feature, check support in the browsers your site needs; support for CSS as a whole does not guarantee support for every feature. W3C: Cascading Style Sheets MDN: CSS
Quick Recap
Key points to remember
- HTML describes a document’s content and structure; CSS describes its presentation.
- A selector identifies elements a rule can match, and a declaration pairs a property with a value.
- Browsers supply default styles, which author CSS can change.
- The cascade resolves competing declarations; specificity is only one part of that process, and source order can decide a remaining tie.
- Some properties inherit from a parent, while others do not.
- CSS evolves through modules, so check support for the feature you plan to use.
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