Yes, I can connect to a DB in CSS

While I appreciate the "shipostedness" in fulfilling the task "connect to DB in CSS" by cramming whole SQL database into CSS custom property and working with it using paint worklet (this would probably not even remotely cross my mind ever), with more pragmatic approach I don't see requirement of having actual database in the CSS in the original tweet, so single boring URL endpoint reading the SQL query from parameter and responding with CSS à la `:root { --result: 'whatever'; }` should accomplish given task as well, without those Houdini's stunts:

    @import "sql.css?query=Robert')%3B%20DROP%20TABLE%20Students%3B--"
    body::before { content: var(--result); }

Or perhaps even more simple with render to image (like `<!..><svg ...><text>whatever</text></svg>`.

    body::before { content: url(sql.svg?query=...); }

If JS and/or strictly static server-side is a must, the sql.css / sql.svg could be handled by a service worker with that sql-asm.js at disposal.

I think of this very informally as having not only for() loops with fixed bounds on the loop index, but also while() loops where you don't know beforehand how many loops it will take to compute something. Alternatively you can express the same thing in terms of recursion. One characteristic is that it's possible to describe computations that never finish. Another is that you cannot always tell from looking at a particular program if it will ever finish.

Exactly how or if the result of a computation becomes apparent to the outside world, I think is not part of the theoretical description.

There's nothing similar in this CSS example, unless I missed it. (I do believe it can be done, the idea is "there" - you need to invent a syntax to declare connection details, error handling, etc; and then you can indeed argue that the .js part is just the "driver")

"ext/mysqli is just a PHP interface for SQL written in C"

What is the difference

Brainf** is famously Turing Complete and can be executed on any desktop you can think of. That alone is enough to prove that your desktop itself is Turing Complete.