Will check (X)HTML Markup on if every opened tag is closed (self-contained tags need to be closed at the end XHTML-Style, e.g. <img .../>, receives an HTML string and returns either true or false.
Currently does not work on HTML single tags - maybe some golfing could resolve this issue.
Skimmed another 2 bytes off by exchanging
!g&&withg||and removed unnecessary;... we're now down to 123 bytes