aria heading level takes precedence - no conflict w <h>

[cliffbert]

atbcb/ICTTestingBaseline#405

https://www.w3.org/TR/wai-aria-1.1/#host_general_conflict:

When WAI-ARIA states and properties correspond to host language features that have the same implicit WAI-ARIA semantic, it can be particularly problematic to use the WAI-ARIA feature. If the WAI-ARIA feature and the host language feature are both provided but their values are not kept in sync, user agents and assistive technologies cannot know which value to use. Therefore, to prevent providing conflicting states and properties to assistive technologies, host languages MUST explicitly declare where the use of WAI-ARIA attributes on each host language element conflicts with native attributes for that element. When a host language declares a WAI-ARIA attribute to be in direct semantic conflict with a native attribute for a given element, user agents MUST ignore the WAI-ARIA attribute and instead use the host language attribute with the same implicit semantic.

User agents testing (Chrome, Firefox) showed the aria-level (instead of native attribute) in accessibility tree when conflict is present between aria-level and H level.