Delighted to see that jQuery 3.0 will have standards-compliant Promises: https://github.com/jquery/jquery/pull/1996
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Here's a scary example from the tree-sitter-c bug tracker: before running the preprocessor, there's no guarantee that C code will parse!
(Typically it will I think, but no guarantees.)
Bash's list of behaviours that aren't POSIX-compliant is longer than I expected: https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html
I used fish for a while and liked it, but POSIX-ish shells work with everything. These days I use a heavily customised zsh.