XHTML Strict only solved one set of leniencies in the web platform, which were also the least important kinds. Malformed HTML makes writing browsers complicated but doesn't generally seem to cause security issues or other visible, obvious, must-fix-now problems.
The real security due to leniency problems in the web platform revolve around the handling of data and how JavaScript works, which were not addressed by XHTML. In that sense it's not surprising it went nowhere.
The real security due to leniency problems in the web platform revolve around the handling of data and how JavaScript works, which were not addressed by XHTML. In that sense it's not surprising it went nowhere.