The root of the problem

So why it's so hard to parse and render SVG?

In my opinion, the main issue is the lack of a reference implementation. No one actually knows how SVG suppose to work. By writing a reference implementation, including a test suite, a lot of issues described in this "book" would never happen.

Instead, we have Chrome. It probably supports around 80% of the spec. Which is not that great, but the best we got so far. And good luck figuring out how it works. It's a huge codebase and even finding the right source file could be challenging.

As for a test suite, SVG 1.1 actually had one. A very rudimentary, but still. And SVG 2 has some SVG-inside-HTML tests, which are impossible to use outside a browser. I have ended up writing my own test suite. It's far from being complete but already includes 1500 tests. Which should give you a hint about the monstrosity we're dealing with.

Asking the SVG Working Group is not an option either. I have contacted them multiple times during resvg development and the most common answer I've got was "we don't know". Which wouldn't be a thing if we had a reference implementation.

In the end, I genuinely believe that it's simply impossible to implement the whole SVG spec. There is just too much of it.

Maybe we should just use something else? You might ask. Like what?
The only open-ish SVG alternative is PDF. Which is a nightmare on its own, while being conceptually simpler, since it doesn't have a text layout, filters/effects, animations and events.