The page http://einsteintoolkit.org/usersguide/UsersGuide.html#x1-108000C1.7.4 describing section C1.7.4 “Interpolation Operators” is showing red invalid markup notices for me (see screenshot).

The offensive code seems to be some MathML (so might be just my browser being picky), most likely (based on Inspect’ing the website):

<!--l. 2689--><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>x</mi> <mo class="MathClass-rel">=</mo><msub><mrow> </mrow></msub></math><span class="obeylines-h"><span class="verb"><mi>o</mi><--

Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2411/invalid-markup-in-userguide-html-docs-for