The most important HTML tool : TIDY
When you are working with HTML, the chance that you would make mistakes in the code is great, and rising with the length of the document. In order to help the HTML coders work with great amount of code, creating from the sloppy and badly-written code a clear and nicely laid markup, David Ragget wrote the HTML TIDY utility.
TIDY can correct the mismatched tags, correct the tags put in the wrong order, and it can perfect the lists that miss some required tags. It practically reduces to zero the work needed to be done by the coder on correcting

