Clarify that viewport meta allows decimal values

[mattgarrish]

As raised in w3c/epubcheck#1481, there are rare occurrences where the value of the viewport meta is expressed with a fractional part. The current definition, however, says the value must be a positive number, suggesting only positive integers are allowed.

We should probably make clear that decimal values are allowed but reading systems may ignore the fractional part.

[iherman]

I would think that is a valid erratum and should be marked as such. Definitely a class 3 in my view.

@dauwhe @shiestyle @wareid @TzviyaSiegman

[bduga]

I am not sure there is a bug here. Decimal seems to imply the base, not that fractions are allowed (decimal vs hexadecimal, etc). "Number" is typically used to mean a whole number or one with a fractional part, "integer" typically means no fraction. For instance, see https://developer.mozilla.org/en-US/docs/Web/CSS/number, but it is pretty common for number to mean whole or fractional.

[mattgarrish]

I don't think it's a bug as much as a minor clarification. Positive number was interpreted for epubcheck as whole numbers absent any other explanation of what we meant there.

I like the idea of just linking/citing the CSS number definition, though. That might also push this down to a class 2 change.

I'll update the pull request, at any rate, and if we decide not to do anything we can just close it off.

[iherman]

With the explicit reference to CSS as the only change, is this really a class-3 change? It smells class-2 change to me...

[mattgarrish]

It smells class-2 change to me...

That's my take now, too. We're just making clearer the connection to the already referenced ICB immediately above the list items.