Add a way to select elements by the amount of classes they have #1211

Prerequisites

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• I tried to reproduce the issue when...
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• I am running the latest version of uBlock Origin (v1.29.0)
• I checked the documentation to understand that the issue I report is not a normal behavior

Description

A steadily higher amount of websites are using randomly generated class names for elements, and a slight majority of such websites also keeps changing those generated class names every month-ish. However, I think I've discovered one thing that can help select specific such elements at least a bit more easily:

On a few of the websites in question, with Twitter and Pixiv as the biggest examples that I personally know of, the amount of classes the elements have are different, and they seem to be fairly consistent.

My wish is that in the setting of the screenshot below, <div class="sc-1nr368f-4 ez9z4y-0 cIk0oy"> would've been selectable with ##div:classes(3), or whatever else you'd want to name it (e.g. :class-number). Likewise it would be selectable with ##div:classes(2) > div:classes(3), and perhaps even ##div:classes(3):has(:scope > section:classes(2)).

Currently, it seems that ##div[class*=" "] can cover everything with 2 or more classes, but I can't find any ways to make it more narrowed-down than that.

An alternative could be to add wildcard support for id and class (e.g. ###*.*.*), similar to what I am currently only able to do with singular elements (e.g. ##*)

A specific URL where the issue occurs

https://www.pixiv.net/en/tags/princess_dress/illustrations (If you're not logged in.)
https://www.pixiv.net/en/tags/princess_dress/illustrations?mode=safe (If you're logged in.)

Steps to Reproduce

1. Visit whichever website, although preferably one with many different class-containing elements.
2. Open the element picker.
3. Attempt to add ##.*, ##div:classes(1) and/or ##div:class-number(1), and see that none of them are accepted.
4. Attempt to add ##div[class*=" "][class*=" "], and see that the amount of successive [class*=" "] has no narrowing-down effect on what is being selected.

Expected behavior:

It becomes possible to select and hide elements based on the amount of classes they have.

Actual behavior:

Currently this can only be done for elements with 0 classes (##div:not([class])) and 1 class (##div[class]:not([class*=" "]). There are currently no known way to specifically select elements by the exact amount of classes they have, if that number is 2 or higher.

Your environment

• uBlock Origin version: 1.29.0
• Browser Name and version: Vivaldi 3.2.1967.41 x64
• Operating System and version: Windows 10 May 2020 Update