We should enable strict units in LESS.
You can do math in LESS code, which is nice. What is not so nice is that you can do math even if the units on the values are incompatible: LESS will happily calculate 20em + 100px and return 120em as a result. This has caused problems (T265560: Text on UploadWizard's "Describe" page is too narrow (due to .mwe-upwiz-data defining "width: calc(-132.5%)"); also some issues caught at the last minute in code review of https://gerrit.wikimedia.org/r/c/oojs/ui/+/514970).
There is an option called strictUnits, which causes it to refuse to compile that. We should enable it. However, since it actually fails the compilation (like a syntax error), it can break some LESS code that has previously worked fine (one example I've seen was a variable ending up with the correct value, but in units of 1/em, due to some fuzzy math).
Here's a task so that folks can complain if that happens.