[css-overflow-5] Remove normative active marker algorithm.

[flackr]

As resolved in #10917:

• 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 [css-overflow-5] What is the active :checked marker when some markers point to elements within different scrolling containers? #11098, and
• uses the writing mode as the primary axis following the proposed primary axis algorithm in [css-overflow-5] Active marker in 2d scroller? #11198.

[flackr]

We probably want the size here and the positions used to account for the optimal viewing region, though perhaps this is a followup?