Validator: Disallow template tags on children of <div submit-success template=...> tags

Related to https://github.com/ampproject/amphtml/pull/12593

We would like to implement a validator rule that allows:

<div submit-success>
  <template>...</template>
</div>

But which disallows:

<div submit-success template=...>
  <template>...</template>
</div>

That is, if the <div submit-success> tag also has a template attribute, then we do not allow any of it’s child tags to be a template tag.

Currently, there is no good way to do this with the validator rule system. My proposed rule type would look like:

tags {
  tag_name: "DIV"
  spec_name: "FORM > DIV [submit-success]"
  attrs: {
    name: "submit-success"
    mandatory: true
    dispatch_key: NAME_DISPATCH
  }
}

tags {
  tag_name: "DIV"
  spec_name: "FORM > DIV [submit-success][template]"
  attrs: {
    name: "submit-success"
    mandatory: true
  }
  attrs: {
    name: "template"
    mandatory: true
    dispatch_key: NAME_DISPATCH
  }
}

tags: {
  tag_name: "TEMPLATE"
  disallowed_ancestor_tagspec: "FORM > DIV [submit-success][template]"
  ...
}

When the validator encounters a <div submit-success> tag, it will record which tagspec was found to match in the tag stack. When it encounters the <template> tag later, it will pass/fail based on which tagspec is recorded in the tag stack.

[honeybadgerdontcare]

Discussed in person, posting for posterity. Another alternative may be to widen the scope of disallowed_ancestor to also handle spec_name in addition to tag_name.

[honeybadgerdontcare]

This has been fixed and will be coming in a future validator release.