implement updated `viewport` meta requirements

[rdeltour]

• viewport meta height and width must not be declared multiple times
• viewport meta may have properties without values

Complements #1400

[rdeltour]

@mattgarrish I'm trying to implement a check for the post-CR viewport rules, notably the requirement that height and width properties are not defined several times, but I'm not sure how EPUBCheck should behave. Can you please help me clarify the following?

The spec says:

the initial containing block [css2] is obtained from the REQUIRED height and width definitions in a viewport meta tag

and

EPUB creators MUST NOT specify duplicate height or width definitions either within a single viewport meta tag or by specifying multiple viewport meta tags.

but it does not explicitly say that there MUST be a single viewport meta tag.

So strictly speaking, the following combinations are valid viewport definitions:

<meta name="viewport" content="initial-scale=1.0"/>
<meta name="viewport" content="width=1200,height=600"/>

<meta name="viewport" content="width=1200"/>
<meta name="viewport" content="height=600"/>

(1) may be safe and working, but I definitely have doubts about (2). Should we fix the spec to disallow that and require viewport is defined in a single meta element, or at least both the (required) width and height values are defined in the same meta element?

I'm hesitant to implement a strict interpretation of the spec (i.e. both (1) and (2) above considered valid); I think it would be more useful if EPUBCheck warned users there, as it is likely an authoring error that would lead to a viewport that may not be parsed by the RS.

/cc @iherman

[mattgarrish]

Reading systems have to ignore all but the first viewport meta declaration, so any permutation that requires compiling the information from multiple tags would be invalid.

[mattgarrish]

but it does not explicitly say that there MUST be a single viewport meta tag.

Ya, I believe that's intentional as whether there are other viewport meta tags isn't specifically an epub concern. Clarifying that only the first tag is processed for fxl purposes, so that's where the dimension declarations need to be, is a useful add. I'll open an issue and PR shortly.