- tag and attribute names must be in lower case;
- for non-empty elements, end tags are required
<!-- BAD -->
<p>paragraph 1<p>paragraph 2
<!-- GOOD -->
<p>paragraph 1</p><p>paragraph 2</p>
- attribute values must always be quoted
<!-- BAD -->
<td rowspan=3>
<!-- GOOD -->
<td rowspan="3">
- attribute-value pairs must be written in full
<!-- BAD -->
<dl compact>
<!-- GOOD -->
<dl compact="compact">
- tag for empty element must end with " />"
<!-- BAD -->
<hr> <br>
<!-- GOOD -->
<hr /> <br />
- Include a space before the trailing / and > of empty elements, e.g.
<br />, <hr /> and <img src="karen.jpg" alt="Karen" />
- do not use empty instance of an element whose content model is not EMPTY (for example, an empty div or paragraph)
<!-- BAD -->
<div></div>
<!-- GOOD -->
<div> </div> <!-- white space inside! -->
<!-- GOOD -->
<div>
</div> <!-- line-break between start and end tags -->
- only five "standardised" named entities are allowed:
& " < > '
Use numeric entities in all other cases (i.e.   in place of , © in place of ©, etc.)
- internal scripts content should be wrapped by
//<![CDATA[ and //]]>, i.e.
<script>
//<![CDATA[
// my scipt here
do_something();
// ...
//]]>
</script>