[css-writing-modes] What are the serialized value when it is "treated as"

The spec defines SVG 1.1 values "treat as" CSS values[1].

When these values are serialized, are they supposed to keep its original
string, or are they supposed to be CSS?

For example, what would we expect to see:
  <div id=test style="writing-mode: tb-rl"></div>
and
  console.log(getComputedStyle(test).style.writingMode);
?

I recall there were some discussions about this for value aliases, but I
don't remember the conclusion.

The same question applies to 'text-orientation: sideways' which is
equivalent to 'sideways-right'.

[1] https://drafts.csswg.org/css-writing-modes-3/#svg-writing-mode
[2]
https://drafts.csswg.org/css-writing-modes-3/#valdef-text-orientation-sideways

/koji