Add code-lang attribute to code element

[extua]

What problem are you trying to solve?

In @edent's recent blog post about the code element he mentions that there could be a gap in the HTML spec for defining the programming language of code in a code block.

For example, I would like to mark up the following code snippet to show that it is Rust code:

fn main() {
    println!("Hello World!");
}

The existing lang global attribute is not suitable for this, as it's supposed to be used for human languages, not programming.

What solutions exist today?

According to the current HTML spec

there is no formal way to indicate the language of computer code being marked up.

But,

authors who wish to mark code elements with the language used, e.g. so that syntax highlighting scripts can use the right rules, can use the class attribute, e.g. by adding a class prefixed with "language-" to the element.

The problem with this is that naming classes with a "language-" prefix is not used by browsers, and as the spec mentions, it's mainly for the benefit of third party styling libraries.

Schema.org has a microdata type for SoftwareSourceCode, but this is overly verbose and again, not used by browsers (or styling libraries).

How would you solve it?

I would propose adding a 'code-lang' attribute to the code element, with this example syntax along similar lines suggested by @edent:

<code code-lang="rust;1.80.1">
    fn main() {
        println!("Hello World!");
    }
</code>

The language codes could be standardised on the pygments language short code list, or another standard coding language identifier? Language version would be specified with a semi colon.

Anything else?

Ideally this (might?) allow browsers to read programming languages in code blocks and add syntax highlighting to the default user agent stylesheet, but I recognise that's a very ambitious outcome for this proposal.

[domenic]

Dupe of #7869.