Who said anything about HTML and CSS?

Apps like VS Code create and manipulate a DOM directly in JS, or more often these days, WASM. To these apps, Electron is just a virtual machine with a lot of browser-like multimedia APIs attached. You pin the exact version of the renderer just like you pin the exact version of your dependencies in a server-side app, or the exact version of the OS in a virtual-appliance VM image.

Electron has nothing to do with "the web" or "web standards" other than borrowing technologies originally developed for those. Electron competes with things like Unity and https://love2d.org/, not with browsers. It's in a category of frameworks with the goal of giving the developer pixel-perfect control over what the user sees, where apps are separately developed, tested, and tweaked, for each target (e.g. each OS, each kind of display/interaction methodology, etc.)

With this category of framework, you don't build your app as generic code targeting a standard; rather, you build, and test, your product against a specific "engine." (Imagine for a moment that people could take Unity apps and run them against any old version of the Unity engine, or even against Amazon's Lumberyard fork of the same. Would anything work?)