The first time was like everyone else does. Google tricks here and there. How to center div, how to create columns with divs, how to do rounded corners etc. This will only get you so far.
The second time around, I took a bunch of courses online, lynda, one of by designers, and bought books. Once you understand the fundamentals of CSS, you can work just fine with css2. Css3 is just an added benefit and great for animation.
It’s true, CSS2 is all you need to build any layout. I actually prefer float grids over flexbox and CSS grid. Float grids are so simple to use in comparison, and all it requires is a list of breakpoints and two to three SCSS mixins, depending on how fancy you wanna get.
But no way I’d write it without modern tooling.
Once you need equal-height boxes in your layout, though, it's time to use something else.
Flexbox and grid do not subscribe to this: flexbox shrinks/grows its content to fit itself. Grid is a configurable beast, something in between, and also makes it possible for grid content (not the container) to define how it itself is laid out.
> The CSS specifications are surprisingly user-friendly! I learned a ton reading the [Flexbox spec](https://www.w3.org/TR/css-flexbox-1/).
The CSS specs are fairly intelligible (as specs go) and once you have enough base knowledge to get a foothold while reading them, they force you into thinking about CSS much more systematically, because that's how specs are written.