Missing string indexer on NamedNodeMap interface in lib.dom.d.ts? #30928

For context, see https://github.com/sindresorhus/refined-github/pull/1783, specifically this commit, https://github.com/sindresorhus/refined-github/pull/1783/commits/48baae0a1952e05b35c3a93d25c854bec2f99b6f.

Also, just posting my tweet about this so that people can find discussions about the issue. See https://twitter.com/nickytonline/status/1117524648982392833

I'll restate what I tweeted as I probably should have posted my question here originally.

The NamedNodeMap interface in lib.dom.ts does not allow for a string indexer, even though vanilla JS supports this in browsers, e.g. someDomElement.attribute['aria-label'].value.

We have code like this in the Refined GitHub extension, so for the time being, I've gone ahead via a declaration merge for NamedNodeMap

interface NamedNodeMap {
      [key: string]: Attr;

I can't tell from the MDN docs for NamedNodeMap if it's standard or not. All they seem to mention is "Attr nodes' indexes may differ among browsers" which wouldn't apply to access by the attribute name.

Just wondering if this was omitted by mistake or is it because this is not considered WHATWG DOM standard? I went to https://dom.spec.whatwg.org/#interface-namednodemap and unless I'm reading it incorrectly, I believe it states that using a string indexer is valid.

Thoughts? Happy to PR this up if it's valid.