This category is for online tools, such as web sites that one visits to have pages checked. Offline tools, i.e. downloadable software, should be submitted to Offline@.
HTML Validators check HTML against the formal HTML specification published by the World Wide Web Consortium (W3C), giving an error report. HTML Lints check HTML for compatibility with different browsers and screen sizes, download times, spelling, and more. If you are only going to use one, use a validator. HTML which follows the HTML specification will be compatible with the widest audience. For compatibility with disabled users, check your documents against the W3C's Web Site Accessibility Guidelines.