Add rule about tabindex > 0 to the styleguide (#12981)

style_guides/html_style_guide.md

@@ -125,4 +125,10 @@ If something is meant to be clickable, favor using a `button` or `a` tag before
 ### Use `tabindex` to make elements tabbable
 
 When added to the tab order, elements become focusable via non-sticky-mode keyboard navigation.
-To add an element to the tab order, you must add an `id` attribute as well as a `tabindex` attribute. If you don't know which number to use for the tab index, or if you simply want to add it to the general flow of the document, use `tabindex="0"`.
+To add an element to the tab order, you must add an `id` attribute as well as a `tabindex` attribute.
+
+You should only use `tabindex="0"` to add an element to the tab flow or `tabindex="-1"` to remove an
+otherwise focusable element from the focus flow (use with care).
+You should never use a value greater than 0, since tabindex is a global counter for the whole
+webpage and not scoped to parent elements, so you would need to manage a globally meaningful order
+across all elements in the whole source code.