Code should always be readable for others. Making use of a strict line indent, comments and DocBlocks make sure everybody is able to understand what the code does and is able to find specific spots faster.
Keep your code valid.
Don't missuse HTML-tags,
put tags into attributes
or try to glitch
an effect through bugs or invalid HTML.
Stick to your HTML standard,
for example closing single tags
in HTML4 and HTML5:
<br> and in XHTML:
Ensure to deliver the same quality of webservice to as much visitors as possible.
Don't use for example
document.location, using the property works
in almost all browsers, but
the correct usage