Identifying and Debugging CSS Errors
In this section, we explore methods to identify errors in CSS code: inspecting code in pages or CSS files and using useful tools to quickly find issues.
Debugging CSS is not as complicated as debugging in other programming languages.
CSS is a forgiving language, similar to HTML, which means that despite syntax or logic errors, the web page will still be displayed.
- If a declaration is invalid, the browser ignores it and continues with the other valid declarations.
- If a selector is invalid, the entire rule is ignored.
The advantage is that users can see the content even if it is not styled correctly. The disadvantage: CSS does not provide explicit error messages, making debugging harder.
Inspecting the DOM and CSS
Most modern browsers provide developer tools, such as the DOM Inspector and the CSS editor, accessible from the developer menu.
In Firefox, the inspector opens with CTRL + SHIFT + I, from the Web Developer → Toggle Tools menu, or by right-click → Inspect Element.
By selecting an HTML element, the CSS panel shows the applied rules. Invalid properties are marked with a warning symbol and crossed out in red. Hovering over the symbol provides a hint about the error.
☞ The checkboxes to the left of each property allow you to enable/disable them for quick effect testing.
Corrections can be made directly in the browser editor, but changes are not saved to the CSS file — they disappear upon page refresh. To copy changes, right-click → "Copy Rule".
CSS Code Validation
The code can be checked using online tools such as:
- CSS Validation Service – validation via URL, local file, or manually entered code.
- CSS Lint – detects errors and best practices.
These services are particularly useful for large CSS files, helping to identify minor syntax errors.