The "Origin" in the CSS cascade refers to where a style rule comes from. There are three primary origins that influence a property's final value: user agent stylesheets, user stylesheets, and author stylesheets. Understanding these origins helps in debugging and predicting style outcomes.
Example 1: User Agent Stylesheet (Browser Default)
/* No explicit CSS written here, but browsers apply default styles */
/* For example, a browser's default might be:
h1 {
display: block;
font-size: 2em;
margin-block-start: 0.67em;
margin-block-end: 0.67em;
font-weight: bold;
}
*/
p {
/* This rule will override any user agent default for paragraphs */
line-height: 1.5;
}
Explanation User agent stylesheets are the default styles provided by the browser. They ensure basic readability for unstyled content. Our line-height rule for p demonstrates overriding a browser's default.
Example 2: User Stylesheet (User Customizations)
/* This would be in a user's custom stylesheet (e.g., in browser settings) */
/* It typically has higher specificity than author stylesheets unless !important is used */
body {
/* A user might set a preferred font family globally */
font-family: "Open Sans", sans-serif !important; /* User preference, often with !important */
}
Explanation User stylesheets allow users to apply their own styles, often for accessibility or personal preference. These are less common for developers to interact with but can override author styles if marked with !important.
Example 3: Author Stylesheet (Your CSS)
/* This is your standard CSS file */
body {
background-color: #f0f0f0; /* Sets a light grey background for the page */
color: #333; /* Sets a dark grey text color for the entire page */
}
a {
color: #007bff; /* Sets the color of all links to a bright blue */
text-decoration: none; /* Removes the underline from all links */
}
Explanation Author stylesheets are the CSS files you write and link to your HTML document. These are the primary source of styling for your website, dictating its visual design.
Example 4: Inline Styles (Author Origin with High Specificity)
<p style="color: red; font-size: 18px;">
This paragraph has inline styles.
</p>
Explanation Inline styles are CSS declarations applied directly within an HTML element's style attribute. They have the highest specificity among author styles and are often used for unique, element-specific adjustments.
Example 5: CSS !important Declaration (Overrides Origin Order)
/* In your author stylesheet */
.critical-message {
color: red !important; /* This rule takes precedence regardless of specificity or origin */
font-weight: bold;
}
/* In HTML: <p class="critical-message" style="color: blue;">This is important.</p> */
Explanation The !important declaration gives a style rule extreme precedence, often overriding even inline styles or rules from a more specific selector. Use it sparingly, as it can make debugging difficult.