In the HTML parser spec, there are 2 instances of the following text:
add the attribute and its corresponding value to that element
The "add the attribute" text does not have a corresponding spec link to
actually specify what to do. We currently use `set_attribute`, which can
throw an exception if the attribute name contains an invalid character
(such as '<'). Instead, switch to `append_attribute`, which allows such
attribute names. This behavior matches Firefox.
Note we cannot yet make the unclosed-html-element.html test match the
expectations of the unclosed-body-element.html due to another bug that
would prevent checking if the expected element has the right attribute.
That will be fixed in an upcoming commit.
