The concept of code smells is admirably applied to CSS in this blogpost. CSS is rarely considered as a "proper" programming language, so it's nice to see it treated with as much rigour as other languages receive.
However, the more I read it, the more I worry that the biggest code smell I come across in CSS isn't even mentioned; worse, it's actually demonstrated, by the very CSS examples and accompanying text. You see, I'm worried that nowhere in the article is a single mention of comments, their use or their abuse. Meanwhile, among maybe 100 lines of quoted code, there are only two copies of the same comment, a comment that adds so little to the reader's understanding that it exemplifies bad comments.
I would suggest if there's one cross-platform, cross-technology "code smell" then it's not commenting your code. But certainly there's a belief I've encountered among frontend developers that good CSS is, by its declarative nature, self-explanatory; that LESS or SASS, combined with judicious attention to nesting, mean that you don't need any accompanying human-readable documentation. Maybe in an ideal world, with only ideal developers involved, that would turn out to be true; but if we're talking about code smells, then: if you discovered this culture in any other programming community, wouldn't you do a double-take?
Of course the brilliant programmer can in theory code without comments; well-written, lucid code can be readable enough that extra human documentation isn't required. But who's that good a programmer, all the time? I'm certainly not, especially when it comes to CSS. Which team consists of such good programmers, and will continue to do so? The best teams I've been a part of have had complementary skills, not all just experts in one technology. What proportion of CSS authors, with a keen eye for visuals and understanding of how to achieve them, are also really good programmers, in the sense that they understand and appreciate dependency, modularity, refactoring, maintainability, etc.... Meanwhile, how does the Dunning-Kruger effect influence the opinions of the very worst programmers, bolstering their belief that they too are good enough to code without comments? How much CSS work is done by designers, or junior developers, who can be skilful in their own fields, but not trained to write good code?
The simplicity and accessibility of writing CSS means that, if we're not careful, we will all end up sitting on the plateau of mediocrity, because a plateau is a comfortable place to be. Rigorous coding, through attention and attendance to code smells, is a great way to start to climb the foothills of expertise. So we can't let the assumptions of the prevailing culture keep us on that plateau: let's train ourselves to comment our CSS, until we're good enough to train ourselves not to.