cwtags #3114946 help writing data- element attributes [validation pending]

Since elt(data-foo=42) is not valid python syntax, let's either:

* replace all _ with -
* or just data_ with data-

prioritynormal
typeenhancement
done in1.0.0
load0.200
load left0.000
closed by#af1b5e590c11 allow writing html5 data- style attributes without pain (closes #3114946)
patchallow writing html5 data- style attributes without pain (closes #3114946) [applied]