zlacker

Unlike typical programs, HTML is often assembled dynamically. This means pages can have broken HTML sometimes, depending on data and context that the developer may not have tested. XML should have been generated from a DOM or something that guarantees proper serialization, but markup-ignorant text-gluing tools are the norm, and they're not capable of ensuring the markup is 100% correct 100% of the time.

These bugs were the worst, because they happened to end users. Users couldn't do anything about unclosed tags or unescaped ampersands, not even notify the developer of the page that refused to display.

Back then HTTPS was rare, but young mobile ISPs loved "optimizing" middleboxes that were messing up the markup. Even if you generated a flawless markup, your pages still wouldn't work for some users. Users were told that your page is bad, and it's your fault, and couldn't contact you about it. ISPs didn't care, because hardly anybody actually used the strict XHTML parsing mode (it made pages inaccessible to IE that had 80% market share). Most "XHTML" pages worked fine thanks to being parsed as regular HTML with invalid extra slashes.