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You really seem to truly misunderstand epubs and the state of the epub reader marketplace. Neither epub 2 nor epub3 is spec'd as the web in a box. They are not HTML, they are instead xhtml docs and the current spec does not call for any specific set of css3 support. That is up to reader implementations just as support for javascript, local site storage, etc, etc.
So very very few e-readers will have support for this set of css3 features. Authors targeting epub should be very aware of the the limitations of the spec and the different degree of implementations mean that there is no guarantee that any e-reading device will show this properly. That includes some of the biggest players in the Market including Amazon Kindle and Adobe.
Second, just because Sigil can not "show it" in its preview, does not mean you can not use Sigil to edit and add that code. You just have to test it someplace else. But that is the case anyway since all e-readers only support a subset of features and they have their own quirks.
Hope this makes things clearer. There will be no "error message" added to Sigil for this very reason.
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