getTotalLength() in the context of textPath element with an href #1178

This is a sub-issue for #1175.
The issue considers adding getTotalLength() to a textPath element.

The proposal says the new members apply to the path attribute. But href is the older, far more common (and currently interoperable) form.

So an author who reads "textPath now has getTotalLength" and calls it on this:

<text><textPath href="#wave">Curving text</textPath></text>

gets 0. Not an error, not a warning.

• And getPathData() gives an empty list,
• and getPathSegmentAtLength() gives null.

The referenced element is itself measurable, so the information is there but not directly accessible, which is what the method is saying.

If path attribute is present but empty, the spec says rendering falls back to href. The API would not follow that fallback. So the text is visibly sitting on a curve while getTotalLength() reports 0.

The rendering and the API would give different results for the same element.

How should this be solved?
If not solved, it would create complexity.