Is there a better way to express this rule?

within amp-nested-menu, allowed elements can contain either one but not both of these attributes: 'amp-nested-submenu-close', 'amp-nested-submenu-open'

Where allowed elements are: heading tags, buttons, or spans.

Right now, we individually list and specify them, but it would probably be easier if we had one rule and a list of tags to which they applied.

Yes. Use attr_lists, defined here. Example usage.

Code:

attr_lists {
  name: "amp-nested-menu-attrs"
  attrs: {
    name: "amp-nested-submenu"
    mandatory_oneof: "['amp-nested-submenu', 'amp-nested-submenu-close', 'amp-nested-submenu-open']"
  }
  attrs: {
    name: "amp-nested-submenu-close"
    mandatory_oneof: "['amp-nested-submenu', 'amp-nested-submenu-close', 'amp-nested-submenu-open']"
  }
  attrs: {
    name: "amp-nested-submenu-open"
    mandatory_oneof: "['amp-nested-submenu', 'amp-nested-submenu-close', 'amp-nested-submenu-open']"
  }
}

Then for each tag element specify it by name:

tags: {
  ...
  attr_lists: "amp-nested-menu-attrs"
  ...
}

And note my code is wrong since the ones below do not have amp-nested-submenu, only the -open and -close

That saves writing out the individual attributes, but I guess we still have to defined the tags one by one?

Yes, there are no grouping of TagSpecs which have specific requirements like this on the tag itself. There are grouping of tag names where it serves a specific purpose, like DescendantTagList.

This is an unfortunate error message since it really should be reporting the issue is it being a descendant of amp-accordion. We should file an issue against the validator on error selection once this is submitted so the issue can point it out.

Filed #25499. Thank you!