User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:99.0) Gecko/20100101 Firefox/99.0

Steps to reproduce:

The CSS round() function would be extremely helpful in avoiding sub-pixel layout mess in text paragraphs when defining line heights in % or similar situations.

Here's the spec: https://www.w3.org/TR/css-values-4/#round-func

I tried to find an existing bug here but the simple search found millions of substrings for "round" and the advanced search found nothing whatever I did. Please feel free to close as duplicate if one exists, the bug search is not working for me (might need fixing).

The Bugbug bot thinks this bug should belong to the 'Firefox::Address Bar' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

This should be relatively straight forward to implement following the Min/Max/Clamp code in here: https://searchfox.org/mozilla-central/rev/d34f9713ae128a3138c2b70d8041a535f1049d19/servo/components/style/values/generics/calc.rs#84

I don't understand much of that code, but one specialty of the round() function is that it can mix units (of a type, e.g. length), for example em with px, or % with em:

div
{
    height: round(2em, 1px);
}

Here's the use case I'm most interested in. It's about setting font sizes and (more importantly) line heights to full pixels from abstract variables that can easily be changed per page:

p
{
    --default-font-size: 16px;
    --default-line-height-factor: 1.4;
    /* would result in 22.4px which round irregularly over multiple lines */

    font-size: var(--default-font-size);
    line-height: round(var(--default-font-size) * var(--default-line-height-factor), 1px)
    /* rather fix that to 22px for even line spacing */
}

Reference to the Chrome bug covering all math functions: https://crbug.com/1284199

(In reply to Yves Goergen from comment #3)

I don't understand much of that code, but one specialty of the round() function is that it can mix units (of a type, e.g. length), for example em with px, or % with em

Yeah, that should just work and is how min() / max() / clamp() works. When they get computed you get pixels, and you might need to preserve them until used value time where you get percentages.

It isn't a one-liner but it should be a matter of extending the min() / max() / clamp() code.

Backed out for bustage on nsStyleStruct.cpp

• backout: https://hg.mozilla.org/integration/autoland/rev/a7488b36c7d6c897fcac1d818faf088552de5c10
• push: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&selectedTaskRun=b2GdmoKzQs6zaKwHRGSu3w.0&revision=c7e10bd5cd69d84bf2f9ed6faef201ea5bfcb949
• failure log: https://treeherder.mozilla.org/logviewer?job_id=395603030&repo=autoland&lineNumber=34473

[task 2022-11-04T21:36:07.039Z] 21:36:07     INFO -  In file included from Unified_cpp_layout_style4.cpp:128:0:
[task 2022-11-04T21:36:07.039Z] 21:36:07     INFO -  /builds/worker/checkouts/gecko/layout/style/nsStyleStruct.cpp: In lambda function:
[task 2022-11-04T21:36:07.040Z] 21:36:07    ERROR -  /builds/worker/checkouts/gecko/layout/style/nsStyleStruct.cpp:3628:7: error: control reaches end of non-void function [-Werror=return-type]
[task 2022-11-04T21:36:07.040Z] 21:36:07     INFO -         }();
[task 2022-11-04T21:36:07.041Z] 21:36:07     INFO -         ^
[task 2022-11-04T21:36:07.041Z] 21:36:07     INFO -  /builds/worker/checkouts/gecko/layout/style/nsStyleStruct.cpp: In lambda function:
[task 2022-11-04T21:36:07.042Z] 21:36:07    ERROR -  /builds/worker/checkouts/gecko/layout/style/nsStyleStruct.cpp:3628:7: error: control reaches end of non-void function [-Werror=return-type]
[task 2022-11-04T21:36:07.042Z] 21:36:07     INFO -         }();
[task 2022-11-04T21:36:07.043Z] 21:36:07     INFO -         ^
[task 2022-11-04T21:36:07.043Z] 21:36:07     INFO -  cc1plus: all warnings being treated as errors

https://hg.mozilla.org/mozilla-central/rev/031200e2a50d

FF108 MDN docs work for this can be tracked in https://github.com/mdn/content/issues/22114. Requires documentation for the method plus updates to the browser compatibility data.

https://hg.mozilla.org/mozilla-central/rev/55fc19ba0ec4

A patch has been attached on this bug, which was already closed. Filing a separate bug will ensure better tracking. If this was not by mistake and further action is needed, please alert the appropriate party. (Or: if the patch doesn't change behavior -- e.g. landing a test case, or fixing a typo -- then feel free to disregard this message)