This method would return an array of CSSPropertyRule, for all the registered properties of the page (via JS and CSS.registerProperty, or CSS and @property --XXX)

Alternatively, this could be a Chrome-only registeredProperties property on CSS, or even be folded into InspectorUtils.parseCSSDeclarations (see Bug 1840320), since we were thinking of also computing declarations properties in it (e.g. isValid, …)