Add the ITS Attribute set to the as an allowed HTML extension item #2732
Description
Section
https://www.w3.org/TR/epub-34/#sec-xhtml-extensions
Describe the problem
The §6.1.3 section on HTML extensions lists some features that can be used in an HTML content document without further ado (ie, without additional definitions like namespaces). Epubcheck checks these and rejects attributes that are not listed in this section. Currently, the (non-deprecated) extensions are the usage of epub:type (epub-type for non-XML) attribute and the RDFa attributes. Additionally to these, though not listed in the sections, the aria-* attributes are also usable the same way (as defined in the HTML specification proper).
However, the Internationalization Tag Set W3C standard defines an attribute set which is not listed in EPUB. These attributes are used, among other things, for the creation and the processing of multilingual content, giving hints for translations, etc. For example, the Requirement for Chinese Text Layout Note uses one of the ITS attributes to switch the text between traditional and simplified Chinese characters, as well as English. The switch is done via a tiny local JavaScript that relies on that attribute. (The example is an HTML page, but it can be converted into EPUB.)
The problem therefore is that an EPUB instance using the ITS attribute is invalid. I believe that is a problem
Describe the fix or new feature you propose
The proposal is to add a separate sub section to §6.1.3 section allowing the usage of all the its-* attributes in an HTML content document.
(Note that other, similar specifications at W3C may come to the fore at some point. We may think about a somewhat more general way of making these extensions possible, without being forced to take each individual extension one-by-one. But we may decide to control this in the future, too, and handling these one-by-one.)
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