Comment by CM30

Personally, I think box-sizing being content-box by default was probably the biggest misstep here, and something that genuinely feels at odds with how 99% of the population would design or build something. The fact IE got it right initially and then it took another few years to even get things fixed across the board just feels utterly ridiculous to me.

Also, not having something like Flexbox or Grid earlier on was another mistake that should have been listed too. The fact we had to resort to hacks related to floats and table layout to create basic layouts was an absolute joke, and indicates that the creators at the time had no idea what a site or application would usually be like design or functionality wise.