I'm marking this as WontFix because I believe the functionality you want is already contained by the calc() function (https://developer.mozilla.org/en-US/docs/Web/CSS/calc). For example, in your demo, you'd just use calc(10px + 10px).

calc() should be supported anywhere a number/dimension is accepted; if you run across somewhere it isn't that's probably a bug which should be filed.

Strictly speaking this isn't the same, but I get your point. It's supported through css, so why adding another option to achieve the same. Well, I would argue my proposal is for developers which want to try out things quickly without less typing (similar to ARROW_DOWN or ARROW_UP key which increase or decrease a value). However, thanks for responding.