CSSStyleDeclaration tests are misleading

[caugner]

The test at https://mdn-bcd-collector.gooborg.com/tests/api/CSSStyleDeclaration/cssFloat assumes that

var el = document.createElement('b');
var instance = el.style;

return an instance of CSSStyleDeclaration, but they now return an instance of CSSStyleProperties.

Could the test check if el.style instanceof CSSStyleDeclaration and return unknown support otherwise?

mdn-bcd-collector/custom/tests.yaml

Lines 1269 to 1272 in ae5de9e

	CSSStyleDeclaration:
	__base: |-
	var el = document.createElement('b');
	var instance = el.style;

Noticed when reviewing mdn/browser-compat-data#27975.