One common problem with CSS and HTML arises when different CSS rules begin to conflict with one another. This can be especially problematic when you are using third party code.

For HTML, validation involves making sure all your tags are properly closed and nested, you are using a [DOCTYPE], and you are using tags for their correct purpose.
CSS has a similar story — you need to check that your property names are spelled correctly, property values are spelled correctly and are valid for the properties they are used on, you are not missing any curly braces, and so on.