When the current frame can be validated, show a status bar icon with the validation controls.
When LibTidy is used, integrate the embedded validator and propose an internal validation of the currently shown document (ie NOT using the online W3 validator, nor any other configured validator).

After the check, the status bar icon is changed into a errors+warnings count.
Also add a very simple (at the moment) view of errors and warnings found.

The look is not the best yet, but it's a start.