W3C is providing free Markup validation service. Using this service we can validate XHTML/HTML syntax of webpages just by giving the URL of the website.
And also, we can validate the syntax by uploading the file or just by pasting the code.
URL of this service is http://validator.w3.org/
W3C is providing free service for checking broken links online and for checking CSS style syntax also.
You can refer this link for seeing complete list of validators provided by W3C.