Writing in a precise language can be tricky, and it is helpful to have some way to find out whether the things we are writing follow the rules. Fortunately, validator tools are available that will look at our webpage and tell us about the issues.
W3C is the standards body that develops standards for the World Wide Web. http://www.w3.org/ is their homepage. Their materials are official and authoritative.
W3C does not try to provide tutorials and guides for novices; instead caters to the detailed and precise needs of experts.
They provide two incredibly useful validators that we can use to find errors in our HTML code or CSS code.
http://validator.w3.org/ is their validator for HTML code.
We can put this somewhere on our webpage to link to an HTML validator.
We can put this somewhere on our webpage to link to a CSS validator.
http://jshint.com/ is a competing validator provided by Anton Kovalyov, a computer programmer from San Francisco.