I do think there’s an important difference from the optionality of curly braces on if statements in many languages: syntax highlighting will normally make it very obvious what is attribute name and what is attribute value, so that any error will be obvious, more obvious than the probable incorrectness of something like `if (a) b; c;` on one line.
Your point on HTML processing is a nice idea, but quite useless in practice for HTML. XHTML failed: people didn’t want to go to the effort of getting it all rigorously correct; they rather wanted the browser to guess what they meant, because it got their intent right most of the time, and now they could forget about various details like tbody elements and trailing slashes. I regularly look at page sources, and I regularly encounter people putting these trailing slashes on various void element; but I can’t remember when I last found a page that actually applied that consistently—invariably they have at least one void element without a trailing slash. My conclusion is that the whole thing is misguided. I would be curious to see the result of attempting to parse all the HTML in something like Common Crawl with an XML parser. I suspect that barely any pages would succeed.
The fact of the matter is that the HTML parsing algorithm is well-defined and very nuanced, so if you’re processing HTML you should use a real HTML parser, and to do anything else is folly—unless you are assiduous about maintaining XML correctness, which you can do, but it’ll be even more of a footgun for others than omitting quotes on attribute values. But if you’re writing something new, then by all means, strongly consider the comparatively simple and principled XML philosophy over the organic and complicated philosophies of the HTML serialisation.
