[fulltrottle]

> The idea with CSS was that we would be able to separate content from presentation

I've been a frontend dev close to 5 years now, and recently I read the entire CSS spec front-to-back in scrutinizing detail for a project.

If the goal was really to decouple presentation and content then CSS has failed spectacularly. Having element selectors, attribute selectors, child and sibling selectors, very tightly couples the content with the layout. It also has the ability to affect content by inserting text nodes for example using the 'content' rule.

I'd say one big gotcha that trips up most people is not understanding when a style is valid or not. Certain styles are only valid for certain display types (see the CSS spec). Also look at the non-normative references on W3C for the default style sheet to see what default rules get applied to elements

To really fully keep content and presentation decoupled you have to stay away from those things and use something like css modules or similarly have a one-class-per-component rule.