get_the_category_list not validate in HTML5

[thomask]

W3C validator shows error on get_the_category_list output thanks to its non-compliant rel. See example:

Line 233, Column 307: Bad value category tag for attribute rel on element a: Keyword category is not registered.

…ěvky z rubriky Tiskové zprávy" rel="category tag">Tiskové zprávy</a></span></p>

Syntax of link type valid for <a> and <area>:
A whitespace-separated list of link types listed as allowed on <a> and <area> in the HTML specification or listed as an allowed on <a> and <area> on the Microformats wiki without duplicate keywords in the list. You can register link types on the Microformats wiki yourself.

#17632, #20333