Seven common CSS Mistakes and How to Avoid Them. They are:

1. Do not use HTML tags to achieve formatting or styling. HTML tags should be used for semantic markup, and CSS attributes should be used to format the content.
2. Do not include CSS formatting directives inside HTML tags (or at least use them sparingly). Use CSS classes.
3. Define common formatting characteristics only once.
4. You can define more than one CSS class for each HTML element.
5. Use hierarchical selectors instead of extra CSS classes.
6. Store all CSS rules in external style sheets. Use the intra-document style sheets only to define exceptions that are local to a single document.
7. If needed, break the CSS rules into multiple files and import the common files into section-specific files with the @import CSS rule.