This official validator is extrememly useful for on the fly (X)HTML validation. If your code contains no errors it is declared valid (well formed) and you are given an option to display your conformity to the W3C's standards with a logo. Be sure to bookmark it. It is essential that you validate your markup.
You can use this form to validate your page in a new window.
Just enter a URI (address) or upload one of your pages and hit the "Validate" button.
You can enter a URI in the Address field below, and have the page validated:
If you want to upload a file from your computer and have it validated, you can use this form: