To put it simply, Pro-CSS on a subreddit specifically just means that the sub/the mods of the sub are opposed to Reddit's possible removal of CSS, which is the formatting system used to make a sub unique (at least on desktop/non-mobile).
More fully, Reddit admins announced recently that they plan to remove CSS in favor of a system that would be consistent over both desktop and mobile browsers. This is mainly because CSS doesn't work in most Reddit mobile apps, including the official one.
There are great concerns among CSS heavy subs that the new system will either be too clunky, or worse, not as customizable as CSS. Not only does this give concerns that every subreddit will start to look more and more alike without being able to differentiate, there are many interesting tricks that subs have used with CSS that may not work in the new system. As well, the removal of CSS would mean that all the work that has gone into CSS-heavy subs would be practically moot and would likely need to be rebuilt from the ground up in a new system which may not support it.
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