Improve focusable area algorithm for element

[CYBAI]

Spec: https://html.spec.whatwg.org/multipage/interaction.html#focusable-area

servo/components/script/dom/element.rs

Lines 1086 to 1105 in 1a4df8f

	pub fn is_focusable_area(&self) -> bool {
	if self.is_actually_disabled() {
	return false;
	}
	// TODO: Check whether the element is being rendered (i.e. not hidden).
	let node = self.upcast::<Node>();
	if node.get_flag(NodeFlags::SEQUENTIALLY_FOCUSABLE) {
	return true;
	}
	// https://html.spec.whatwg.org/multipage/#specially-focusable
	match node.type_id() {
	NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLAnchorElement)) |
	NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLInputElement)) |
	NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLSelectElement)) |
	NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTextAreaElement)) => {
	true
	}
	_ => false,
	}
	}

In the current implementation, we only make anchor, input, select and textarea be focusable.
Thus, I think we can improve the algorithm of this method to be up-to-date with spec.