@Hawke Nowhere is it written that the word should be capitalized before its transformation, be it a contraction, an elision, aphetism or anything else.
If it was so, it would be clearly specified as it has numerous non obvious implications, like:
* "bout" ('bout is sometimes written without the leading apostrophe) should not be capitalized, even as a first word (i.e. in "bout Time" or "bout Dat Time"). Weird.
* Contractions like "shouldn't", "couldn't", "don't",... should end in "N't", with a capitalized N. Weirder.
* Any backslang word, like "yob", should actually be written uncapitalized with an ending capital, like yoB? (obviously not, just a thought-provoking question)
As I read it in the guidelines "capitalize contractions and slang consistent with the rules above" means "capitalize the resulting word", that is:
- Shouldn't, not ShouldN't
- Yob, not yoB
- and of course 'Bout, not 'bout, with or without a leading apostrophe