Before this change, only keyword list literals could be passed to
elements. If they had non-literals as values, then those would compile
to EEx expressions.
This allows a non-literal to be passed as attrs and have the entire thing
compile to an EEx expression, which will pass the non-literal to a
"runtime_attrs" function, which evaluates a keyword list into a safe
string.
That last part might need to be reworked if the user is not using
the Phoenix.HTML.Engine EEx Engine.
