> 6. Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single U+002F SOLIDUS character (/). This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
The problem with making the SOLIDUS optional for so-called void elements is that the set of void elements isn't finite across time. A new one could be added in the future, which means any document which relies on implicit syntactic behavior requires an updated parser simply to get the most basic AST.
XML and XHTML formalized a distinction between syntax from semantics, permitting forward compatibility for code, like low-level parsers, only processing the syntax.
The WHATWG made the argument that out in the real world syntactically correct documents are almost the exception, not the norm. Because that's true the vision of being able to ubiquitously slice-and-dice documents with a shared syntax but distinct internal semantics was not attainable as a general matter. Any software consuming HTML out in the open universe would always need to be aware of contemporary HTML semantics even for low-level parsing. The insistence on separating syntax from semantics for HTML had a high cost but very little realized benefit.
However, the benefits are attainable within a closed universe, such as a CMS. And this is why HTML5 doesn't require, but nonetheless permits, XML- and XHTML-compliant syntax. It's not even treated as an error or exception, not in the way that other malformed but recoverable constructs are. A self-closing tag is syntactically valid, so there's absolutely no reason not to use it other than convenience. Excluding it out of convenience is perfectly acceptable, but in some situations--e.g. when using the more general and diverse ecosystems of XML and XSLT processors--it can be extremely inconvenient to exclude the SOLIDUS.
From https://html.spec.whatwg.org/#start-tags
> 6. Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single U+002F SOLIDUS character (/). This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.
From https://html.spec.whatwg.org/#void-elements
> Void elements: area, base, br, col, embed, hr, img, input, link, meta, param, source, track, wbr
The problem with making the SOLIDUS optional for so-called void elements is that the set of void elements isn't finite across time. A new one could be added in the future, which means any document which relies on implicit syntactic behavior requires an updated parser simply to get the most basic AST.
XML and XHTML formalized a distinction between syntax from semantics, permitting forward compatibility for code, like low-level parsers, only processing the syntax.
The WHATWG made the argument that out in the real world syntactically correct documents are almost the exception, not the norm. Because that's true the vision of being able to ubiquitously slice-and-dice documents with a shared syntax but distinct internal semantics was not attainable as a general matter. Any software consuming HTML out in the open universe would always need to be aware of contemporary HTML semantics even for low-level parsing. The insistence on separating syntax from semantics for HTML had a high cost but very little realized benefit.
However, the benefits are attainable within a closed universe, such as a CMS. And this is why HTML5 doesn't require, but nonetheless permits, XML- and XHTML-compliant syntax. It's not even treated as an error or exception, not in the way that other malformed but recoverable constructs are. A self-closing tag is syntactically valid, so there's absolutely no reason not to use it other than convenience. Excluding it out of convenience is perfectly acceptable, but in some situations--e.g. when using the more general and diverse ecosystems of XML and XSLT processors--it can be extremely inconvenient to exclude the SOLIDUS.