A bare string
A tag
A
tag that supposedly has contents.
AT&T
Within a textarea, markup like <b> tags and <&<& should be treated as literal
This numeric entity is missing the final semicolon:
, that attribute value was closed by the subsequent tag
a
This document contains (do you see it?)
This document ends with That attribute value was bogus
The doctype is invalid because it contains extra whitespace

That boolean attribute had no value

Here's a nonexistent entity: &#foo; (do you see it?)
This document ends before the entity finishes: >

Paragraphs shouldn't contain block display elements, but this one does:

you see?
Multiple values for the same attribute.

Here's a table

Here's a nested table:
foo

This tag contains nothing but whitespace:

> This p tag is cut off by

the end of the blockquote tag

This table contains bare markup

\n This link is never closed.\n
\n
\n
\n This link is closed.\n
\n
This document contains a surprise doctype
Mixed case tags are folded to lowercase
Tag name contains Unicode characters
Attribute name contains Unicode characters