validator.py fails to detect html errors

[mwetter]

Some html errors are not detected when running

../bin/runUnitTests.py --validate-html-only

For example, use the IBPSA library, commit 93ced3c8873008aac54a500951554adcf8c0c9cb, and change

$ git diff
diff --git a/IBPSA/Media/Refrigerants/R410A/specificIsochoricHeatCapacityVap_Tv.mo b/IBPSA/Media/Refrigerants/R410A/specificIsochoricHeatCapacityVap_Tv.mo
index 3c6a9a015..ac4271f66 100644
--- a/IBPSA/Media/Refrigerants/R410A/specificIsochoricHeatCapacityVap_Tv.mo
+++ b/IBPSA/Media/Refrigerants/R410A/specificIsochoricHeatCapacityVap_Tv.mo
@@ -42,7 +42,7 @@ Function that calculates the specific isochoric heat capacity
 The specific isochoric heat capacity is evaluated from the partial derivatives
 of the Martin-Hou equation of state.
 </p>
-<h4>References</h4>
+<h3>References
 <p>
 F. de Monte. (2002).
 Calculation of thermodynamic properties of R407C and

The line https://github.com/lbl-srg/BuildingsPy/blob/master/buildingspy/development/validator.py#L160 is executed but no error is triggered.