PDF is a mess because it's an ancient originally-proprietary format based on a proprietary language and the Adobe "standard" didn't actually specify a lot of important things for a long time, leaving other implementations to try to reverse engineer them.
Meanwhile you look at actual standard formats like PNG or UTF-8 or, as you say, HTML, and the implementations generally do a lot better because there is a proper spec.
Meanwhile you look at actual standard formats like PNG or UTF-8 or, as you say, HTML, and the implementations generally do a lot better because there is a proper spec.