Manipulating it is very slow, because of the way it is designed. It is not intended for building dynamic GUIs, it's for laying out text high quality text, once, on pageload. This is the other problem, the box model (even flexbox) is just not suited for designing GUIs.
So few websites are pure text these days, you inevitably end up needing loads of GUI widgets and the DOM is simply not suitable for laying them out in a sensible way. It takes ages, and css layout 'rules' are so hard to predict. You shouldn't have to be a guru to just put some fucking boxes in a row.
Try building a desktop app using any mature constraint-based layout system, and you'll see what I mean.
As you can see, pretty much every css framework is actually a hacky attempt to replicate a GUI layout pattern from 20 years ago, and you have to turn backflips in order to convince the DOM to do it performantly.
So few websites are pure text these days, you inevitably end up needing loads of GUI widgets and the DOM is simply not suitable for laying them out in a sensible way. It takes ages, and css layout 'rules' are so hard to predict. You shouldn't have to be a guru to just put some fucking boxes in a row.
Try building a desktop app using any mature constraint-based layout system, and you'll see what I mean.
This is how java does it: https://docs.oracle.com/javase/tutorial/uiswing/layout/visua...
As you can see, pretty much every css framework is actually a hacky attempt to replicate a GUI layout pattern from 20 years ago, and you have to turn backflips in order to convince the DOM to do it performantly.