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Supporting calc(100% + 0.333em) in the width attribute of <mpadded> #306
Description
<mpadded> is only really useful for horizontal spacing when it's possible to add something to the width. For example, if one sets lspace on <mpadded> but does not change the width, then the lspace eats into the width of the content.
MathML 3 had the width="+0.333em" notation for this (with + as the prefix) but this was apparently replaced by a new notation in MathML 4, see this example:
<mrow>
<mi>x</mi>
<mpadded lspace="0.3em" width="calc(100% +0.6em)">
<mi>y</mi>
</mpadded>
<mi>z</mi>
</mrow>The problem for MathML Core here is that the 100% here refers to the width of the child, and not to the width of the parent, as is the case in CSS.
But there is actually a solution in CSS, with the newly proposed calc-size function. The following actually works correctly in Chrome (which is the only browser currently implementing calc-size):
<math display="block">
<mi>x</mi>
<mpadded lspace="0.16667em" style="width: calc-size(min-content, size + 0.33333em)">
<mi>sin</mi>
</mpadded>
<mi>θ</mi>
</math>So, I think the spec should be either changed so that:
widthon<mpadded>accepts any CSS value (currently only allowslength-percentage), so that the newcalc-size()can be used there- support for
calc(100% + 0.333em)is explicitly hard-coded in the spec and is internally translated tocalc-size() - (as David says below)
widthacceptscalc-size()in addition tolength-percentage
Activity
davidcarlisle commentedon Jan 6, 2026
Thanks for this, It means that the example in the MathM4 draft is wrong (and I just confirmed it is as you say) so we should change that to use calc-size (probably with a warning that it's not generally implemented) I'm a bit confused as it's been in the draft for quite a while I'm sure we tested that at the time, but perhaps we were confused by the implementation status.
so the question would be whether we should allow length-percentage or calc-size everywhere that we currently allow length-percentage or just on mpadded (doing it for all length attributes seems more consistent)
I'm not sure we need to do that, it seems just to be a spec error (in mathML4 draft) not something that needs to be supported.
dginev commentedon Jan 27, 2026
During the MathML Core meeting yesterday this issue was discussed and I encountered the percentage widths being ignored in Chromium. I have now reread the related discussion in #183 that motivates that behavior.
Here is a codepen example of what I witnessed:
https://codepen.io/dginev/pen/yyJPmBQ
Writing that in here, since I assume an mpadded width of
calc()based on a percentage value will not be well-founded until #183 is resolved in some way.tmke8 commentedon Jan 27, 2026
Basing it on
calcwill indeed not work, as I show above in the first example. But, as I wrote after that, it does work when it's based oncalc-size(). Your example works then:davidcarlisle commentedon Jan 27, 2026
@tmke8 yes calc-size works but there was feeling it was too early to suggest that as a general replacement at the current time as its implementation status is uncertain. We can always come back to that,
davidcarlisle commentedon Jan 27, 2026
@dginev I missed this during the meeting, but percentages not working for
widthis actually specified behaviour in core.https://www.w3.org/TR/mathml-core/#inner-box-and-requested-parameters