Include invalid outputs in the table with valid versus invalid parser inputs

[alwinb]

What is the issue with the URL Standard?

At the start of section 4. URLs, there is a table that illustrates various combinations of input– and base URLs. The table expresses if the combination of the input and the base is considered valid; and what the resulting output is.

I propose to include examples that produce an invalid output URL in addition to those that cause Failure, and indicate this clearly in the table.

Maybe this can be done by adding another column with ✅ and ❌ markers for the output.

I think it would be useful to also call out the following phrase just above the table into a separate block:

The output of the URL serializer is not always a valid URL string.

Either in one of the green Note blocks, or more appropriately,
in the red with the red outline.

(As a bonus, we can include footnotes of some sort that link the the various validation issues, though it might be tricky to get a nice layout).

Motivation:

The differences between valid, invalid and failure, are often a source of confusion. Especially the fact that the parser can Produce invalid URLs can be counter intuitive.

This issue was taken out of the discussion in #379, cc @domenic