I also mourn the loss of a simplicity that meant that curious teenagers (like myself way back when) could look at a site's code and easily learn from it.
The use of CSS and divs instead of tables for layouts has had an interesting side effect: most elements of a page have IDs or classes. This makes it far more obvious what they are there for without having to use a web page inspector.
That said, the continuing expansion of HTML, CSS and JavaScript APIs worries me too, but for a different reason. I'm worried that the stack will become so big that there is no possibility of new rendering engines or browsers.
I've done front-end stuff on and off for a few years and I've always thought variables would be the "easiest" thing for implementers to enhance CSS.
This lack of variables in CSS is already being solved in a nonstandard way. Might as well standardise a solution.
css variables, transforms, gradients, border-radius, multiple background images, etc.
... you'll be glad to know that those features are available as well and that you don't have to add complexity to whatever you're building to hack those in. I also don't see how CSS variables, or other recent CSS features, hinder anyone from learning from a site's code.
All these things make it easier to create stuff. My only complaint is that they're not added soon enough.