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Currently the wording under 4.8.2.1.12 General guidelines is "The most general rule for writing alternative text is that the intent is that replacing every image with the text of its alt attribute not change the meaning of the page." It's probably been written in this way to avoid references to words in RFC2119, but in my opinion would be better written as "The general rule when writing alternative text is to consider that the intent is to allow every image to be replaced with the text of its alt attribute without changing the meaning of the page" would be clearer.
I've tweaked the text, though not quite in the way you suggest. Let me know if you think the new text is still problematic.
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