When writing HTML by hand, these are the most common mistakes that lead to invalid HTML or XHTML:
Have a WordPress blog? Having trouble with Section 508 and validation? Get the WordPress XHTML validator, and forget about validation issues!
Forgetting to terminate entities
The & character is reserved in HTML and XHTML. You can't just place it on your code and expect it to be valid.
The & character is used to post entities in content. Entities represent special characters, such as í (that would be í). When writing an entity by hand, a common mistake is to forget the trailing semicolon. That unterminated entity is shown as a series of errors in the W3C Validator.
And how do you put a literal ampersand? Use
Ampersands in URLs
This is actually a variant of the former error. Basically, even if it's text on an URL (for example, in an
href attribute on an
a tag), you can't just put an ampersand there. You need to use the proper form (see above)
Neglecting to properly close a tag
In XHTML, some tags, like <br>, need to be closed inline. The tag should be written like this:
Overlapping and context-sensitive tags
Some tags cannot go within others. For example, it is an error to include a
<p> tag within another
<p> tag. It is also illegal to start an
<i> tag, then place any block-level tag (such as
<p>) within it. Another common mistake is closing a tag before its children tags have been closed.
Watch out for errors, and validate your pages often! So long, folks!