You've touched on a slightly interesting topic. HTML has never had strict parsing requirements, for example, it was completely permissible in HTML4 to leave a t

You've touched on a slightly interesting topic. HTML has never had strict parsing requirements, for example, it was completely permissible in HTML4 to leave a tag unclosed if you subsequently opened a tag which could not be nested (as in your example of two <li>s in a row). Containers have always been implicit in most cases, so it's perfectly acceptable to start a table row without having started a table, the parser will start the table when it encounters the row. This is ultimately because HTML is derived from SGML, a markup standard that is visually similar to XML but with very relaxed parsing. However, HTML4.1 for a time faded in popularity compared to XHTML, which was an "HTML-like" markup derived from XML rather than SGML. XML has significantly stricter parsing rules, for example always requiring full closing tags, trailing slashes on self-closing tags, etc. XHTML's popularity seems to have been in large part a result of the IE6 backlash. Problems with web standards were widely perceived as a high priority for the web. This was the time period in which people kept putting "W3C Compliant HTML" badges in their footers. Because XHTML had stricter parsing rules, it fit into the general scheme of emphasizing web markup that was "correct" by allowing for strict automatic validation. Because it was XML-compliant it also fit nicely into both the popular-at-the-time "Semantic Web" (e.g. XHTML documents could be validated against XML DTDs/schemas) and dovetailed with browser support for XSLT (in which case a document could be sent to the browser as semantic XML with an XSLT "style sheet" for transformation to XHTML for presentation), which was one of those things that seemed like "the future" for about five minutes. I had done my personal website that way for a hot minute. XHTML's vogue was somewhat short-lived, while it was a big part of the "web" scene in e.g. 2007, it was largely forgotten the moment HTML5 because available. What seems to have survived from XHTML, though, is a much stricter approach to writing SGML being seen as more or less required. I think a lot of people "grew up" on XHTML, quite possibly unknowingly, and so errantly view HTML itself as XLM with the subsequent parsing rules. As always, there is often value in being explicit and verbose in markup in terms of maintainability. But in a lot of cases readability and maintainability are not harmed or even improved by saving some typing... as in cases like lists and tables. That said, a big part of the phenomena is that hand-writing or reading HTML is decidedly out of vogue today, and so in general the readability quality of most HTML on the web is extremely poor, generally a result of it being generated by templating or composable component systems which often abandon basic indentation, as well as by the popularity of frameworks with unusual use of HTML such as non-semantic class-driven CSS frameworks. Of course this phenomena isn't entirely new, FrontPage and DreamWeaver were popular in their day and generated some pretty terrible markup, but it seems to have become more common, rather than less, for HTML to be machine-generated and thus extremely unpleasing to humans.