Sometimes when working with CSS and / or XHTML (like when creating a new wordpress theme) we will want to validate any changes we have made. I have provided a pair of tools that allows for just that. Just copy the text from the file you want to validate into these tools and *poof* it will tell you if there are any errors.
A markup validator service – http://validator.w3.org/
Note: You can provide the url, upload the file, or just copy and paste the text of the file.
A CSS validator service – http://jigsaw.w3.org/css-validator/
Note: Same as above