Can I add a custom attribute to an HTML tag?
You can add custom attributes to your elements at will. But that will make your document invalid.
In HTML 5 you will have the opportunity to use custom data attributes prefixed with data-.
You can amend your !DOCTYPE declaration (i.e. DTD) to allow it, so that the [XML] document will still be valid:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
[
<!ATTLIST tag myAttri CDATA #IMPLIED>
]>
#IMPLIED means it is an optional attribute, or you could use #REQUIRED, etc.
More information is in DTD - Attributes.
No, this will break validation.
In HTML 5 you can/will be able to add custom attributes. Something like this:
<tag data-myAttri="myVal" />