[cssom] serializing U+0000 NULL

I'm adding a selector escape function to Sizzle (the selection library
backing jQuery), and originally planned to match
https://drafts.csswg.org/cssom/#the-css.escape()-method but ran into a snag
regarding U+0000 NULL [1]. The draft calls for throwing an
InvalidCharacterError exception in §2.1, but that doesn't fit with our
library. It doesn't seem to fit within CSS, either, though. \0 is a valid
escape per CSS Syntax [2], although it is returned as U+FFFD REPLACEMENT
CHARACTER, and all major browsers but one now respect it
(e.g., jQuery("<div><span
data-attr='&#xFFFD;'/></div>")[0].querySelector("[data-attr='\\0 ']")
returns the span)—the holdout is Safari, in which [data-attr='\\0 '] is
treated as valid but never matches anything.

So I'm wondering if you'd be willing to escape NULL rather than throwing an
exception. And if not, perhaps you could shed some light on why the
exception was added in the first place?

A brief history of NULL:
* 2010 dbaron includes NULL in proposed escaping language (presumably as
"\0 "): https://lists.w3.org/Archives/Public/www-style/2010Feb/0162.html
* 2011 CSSOM WD includes escaping NULL:
http://www.w3.org/TR/2011/WD-cssom-20110712/#common-serializing-idioms
* 2012 (not directly related) Tab suggests a codification in input
preprocessing of replacement with U+FFFD REPLACEMENT CHARACTER:
https://lists.w3.org/Archives/Public/www-style/2012Oct/0687.html
* 2013 CSSOM WD introduces InvalidCharacterError exceptions for escaping
NULL (rethrown by CSS.escape):
http://www.w3.org/TR/2013/WD-cssom-20131205/#common-serializing-idioms
* 2015 no change: https://drafts.csswg.org/cssom/#common-serializing-idioms

[1] https://github.com/jquery/sizzle/pull/364#discussion_r44619782
[2]
http://www.w3.org/TR/2014/CR-css-syntax-3-20140220/#consume-an-escaped-code-point0
and http://www.w3.org/TR/2014/CR-css-syntax-3-20140220/#escaping