Mandatory elements

Check that each Web page has a valid source code according to the document type, a relevant title and a default human language identification. Check that tags are not used for presentation only, that changes in human language and changes in the reading direction are specified.

Synthesis

Declare a valid doctype.
Always declare the language of the page on the <html> tag.
Make sure you have a relevant page title.
Indicate the changes in language and reading direction.
Always validate your source code.
Use web semantics correctly.