Error Handling

Unlike programming languages such as JavaScript or Python, CSS is designed to be resilient. If a browser encounters a line of CSS it doesn't understand either because of a typo or a property it hasn't implemented yet it won't crash the website. Instead, it simply ignores the "broken" parts and moves on to the next valid rule.

This is a double-edged sword: your site stays functional, but layout bugs can be incredibly tricky to track down because the browser won't throw an error in the console.

1. The "Skipping" Mechanism

When CSS finds an error, it searches for the next valid "re-sync" point. This is usually a semicolon ( ; ) or a closing brace ( } ).

If you omit a semicolon, CSS treats the current line and the next line as a single, giant, invalid instruction.

Interactive Error Demo

2. Common CSS Errors to Watch For

Because CSS fails silently, you need to keep an eye out for these frequent culprits:

Common CSS Errors
Error TypeWhat Happens
Missing SemicolonThe current rule and the next rule are ignored.
Invalid PropertyThe browser ignores just that specific line (e.g., colour: red instead of color).
Unclosed BracesIf you forget a }, all CSS following that block will likely be ignored by the browser.
Spelling Errorsbackround: red; will be ignored completely.

3. How to Debug CSS

Since the browser doesn't alert you to errors, you must use external tools and built-in features:

  1. Browser DevTools: In Chrome or Firefox, if you inspect an element, invalid properties will often have a strikethrough or a yellow warning triangle next to them.
  2. CSS Linters: Tools like Stylelint or CSSLint scan your code as you write and highlight syntax errors immediately in your code editor (like VS Code).
  3. W3C Validator: You can paste your CSS into the official W3C validator to check for compliance.

Key Takeaways

  • Resilience: CSS is built to fail gracefully. One error won't break the entire page.
  • The Semicolon Rule: Forgetting a semicolon is the most common "invisible" error. It "kills" the current line and the one immediately following it.
  • Silent Failure: Use Browser DevTools to look for "crossed-out" properties this is the only visual clue the browser gives you that a rule was rejected.
  • Syntax Checkers: Use a Linter in your code editor to catch typos and missing symbols before you even save your file.