Could not find the language 'HTML, XML'

[Antecer]

if lang is xml, the language name will result HTML, XML

function test(code, lang) {
    let language = hljs.getLanguage(lang)?.name ?? 'plaintext';
    return hljs.highlight(code, {language}).value;
}

Then i will get this error:

if i replace HTML, XML to xml, it will work normal:

function test(code, lang) {
    let language = hljs.getLanguage(lang)?.name ?? 'plaintext';
    if (language == 'HTML,xml') language = 'xml';
    return hljs.highlight(code, {language}).value;
}

[joshgoebel]

if lang is xml, the language name will result HTML, XML

This is correct. You are supposed to pass the shortname (ie, xml) to highlight, not the name attribute... the name is only for completness.