MathML Core should mention ElementCSSInlineStyle for the IDL style attribute

[fred-wang]

See https://w3c.github.io/mathml-core/#dfn-style
https://w3c.github.io/mathml-core/#dom-and-javascript

It refers to HTML which itself refers to CSS (https://html.spec.whatwg.org/multipage/dom.html#the-style-attribute):

The style IDL attribute is defined in CSS Object Model. [CSSOM]

Maybe this sentence can be copied into the MathML spec too, so it's more obvious.

We can also probably remove "tentative" for these tests:
mathml/relations/html5-tree/css-inline-style-dynamic.tentative.html
mathml/relations/html5-tree/css-inline-style-interface.tentative.html