1.5. HTML, XML and other markups
Unlike other files, indentation for XML/HTML is 2 spaces.
Don’t use inline HTML style attributes. Only CSS classes.
Use HTML5 doctype:
1.5.2. CSS interaction
Make sure all elements that are used in JS have an ID. This helps with testing.
Make sure all related elements have the same class.
Make sure that a page will not contain the same ID more than once.