"Document" formats

Compared to HTML files, document formats have basically no advantages. The biggest disadvantage is that you need specific applications to read them. You can't open a PDF in a text editor; you need a "PDF viewer". And while most browsers can render PDFs natively, they shouldn't have to, because HTML exists. That browsers have to implement PDF support Notice the commit count on that repository. How many thousands of hours do you think were put into it? I cloned it and counted the source lines at well over 100k. as well as HTML adds to the complexity and bugginess of browsers (as well as the difficulty of new browsers entering the market).

Making matters worse, document formats besides PDF have compatibility issues. Unix operating systems don't usually come with viewers for them preinstalled. PDFs are not the worst but stuff like .xlsx or .ppt tends to require installing something absurdly heavyweight like libreoffice.

Honestly, half of the time I see PDFs used on the web, there's no real reason it couldn't have been a plaintext file, let alone an HTML file.

"Smart" quotes and unicode dashes

While almost everything supports UTF-8 these days, it's still not everything, and even if it were, why would you want to use a character that can't be directly typed on your keyboard? Both are unnecessary, a chore to type, and risk compatibility issues (in fact smart quotes have been the cause of a failure in an enterprise app I develop).

XML, in any application other than HTML and SVG

The only reason I qualify it that way is because instituting an alternative to HTML would be an incredibly massive undertaking; even just to coexist alongside it we would have to get support implemented in all the browsers and everything. The idea of SVG (vector graphics in a human-readable format!) is glorious honestly; my only gripe with it is that it's implemented in XML. I dream of standardizing a non-XML version of SVG, but for the time being, I consider SVG one of the few justifiable uses of XML.

JSON should replace XML in the vast majority of uses. For web applications, JSON's inherent affinity with Javascript is an enormous plus, and support for it is also in the stdlib of most other languages.

(Also, I know that HTML isn't technically XML, but it is deeply influenced by it.)

Flaws of XML:

PHP

I still feel a bit uncertain putting a language I've never personally used in this pile. I won't say that everyone should stop using C++ and Java despite all the negative things I've heard about them (although with the information I have it's very probably true that they should). I have a bit more concrete information about PHP. After hearing many other devs speak offhandedly of its horrors, I took to Google to find out what it was all about, and I read PHP: a fractal of bad design. If everything that guy says is true, even minus all the ones he notes are fixed in more recent versions, that anyone would voluntarily use PHP just boggles my mind. (There's also phpsadness.com.)

I didn't want to make a big judgement off hearsay from the critics alone, so I made a dev.to post addressed to PHP devs. The responses I got made me a lot more sure. Not a single PHP defender raised a single point that was even about the design of the language. They all said things like "I use PHP because with PHP I can make websites" or "Well PHP isn't as bad as PHP used to be". One response was so revealing it was slightly comical, so I'll paste it here:

I do accept that PHP lacks some features that other languages have but that doesn't mean PHP is a bad language. PHP lets you write your code in the way you want which can be an advantage or a disadvantage according to one's knowledge of software architecture. The other reason that I use PHP is the reason that deploying it is very easy thanks to many hostings supporting PHP.

The reason I find this so cringey is that when someone interprets any criticism (I linked the Eevee article), even criticism mostly about confusingness, as "it lacks features", it tells me that they have a very bad way of thinking about how software should be designed.

The one relevant point I've heard raised is that it apparently has optional static type hints. That's a massive plus over something like Python, but nowhere near the babel tower of flaws it seems to have.

So that's why despite not knowing PHP, I'm reasonably confident saying it's awful and we should abandon it.