removes the normative algorithm for determining the active marker and leaves it up to the UA,
additionally defines an example algorithm that takes into account improvements mentioned in the issue for selecting the next marker when it is nearer to being in view.
The example algorithm also
includes recursive marker selection within nested scroll containers as proposed in #11098, and
uses the writing mode as the primary axis following the proposed primary axis algorithm in #11198.
As resolved in #10917:
The example algorithm also