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You can follow HTML standards by analyzing errors and warnings caused by specific pages by using a Firefox add-on HTML Validator (https://addons.mozilla.org/en-us/firefox/addon/html-validator/). It has the option to check HTML and CSS validation separately online using a w3.org tool or by using W3C's free online validator tool (http://validator.w3.org/).

Results may vary from one tool to another due to analyzing error pattern difference. <br> may not be erroneous under HTML Tidy but it's an error under the W3C validator as it should be <br />.

In the following screenshot, you can see the result from HTML Validator for the URL www.packtpub.com:

While the following screenshot, shows the result from W3C's online validator for the same www.packtpub.com URL:

The preceding screenshot shows that there are 27 errors and 2 warnings that exist and should be taken care of.

Try W3C's online validator for your own website and try and fix all the errors and warnings shown by this tool.