This is extracted from bug #683896 as it's a similar issue.

The spec seems pretty clear regarding this [1]:
> User agents must only consider the following pieces of text as checkable for the purposes of this feature:
> *  The value of input elements whose type attributes are in the Text, Search, URL, or E-mail states and that are mutable (i.e. that do not have the readonly attribute specified and that are not disabled).
> *  The value of textarea elements that do not have a readonly attribute and that are not disabled.

[1] https://html.spec.whatwg.org/multipage/interaction.html#spelling-and-grammar-checking

 

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