Use explicit units

[fred-wang]

For example linethickness="0" should be linethickness="0px"

[mazeto]

Or linethickness="0.1em" for compatibility with hidpi displays.

[fred-wang]

cc @distler

All unitless length values are discouraged in MathML 3: "A number without a unit is intepreted as a multiple of the reference value. This form is primarily for backward compatibility and should be avoided, prefering explicit units for clarity. " (https://www.w3.org/TR/MathML3/chapter2.html#fund.units)

However, "0" is likely to be preserved in MathML Core: w3c/mathml#24 ; still I think it is safe to use "0px" everywhere so I'll land a commit for that.

[fred-wang]

Or linethickness="0.1em" for compatibility with hidpi displays.

Using a nonzero value is incorrect, for example the layout of fraction is different depending on whether linethickness is zero or not: https://mathml-refresh.github.io/mathml-core/#mfrac ; also I think some AT rely on that to present fraction differently.