Selectors such as a[href=#main] are not valid CSS syntax because the value contains special characters that are not quoted. Until jQuery 1.11.3/2.1.4 this was accepted, but the behavior is non-standard and was never documented. In later versions this selector throws an error. In some cases with complex selectors, Migrate may not attempt a repair. In those cases a fatal error will be logged on the console and you will need to fix the selector manually.
Put quotes around any attribute values that have special characters, e.g. a[href="#main"]. The warning message contains the selector that caused the problem, use that to find the selector in the source files.
Examples of incorrect code for this rule:
$('[href=#main]')
var selector = '[href=#main']Examples of correct code for this rule:
$('[href="#main"]')
var selector = '[href="#main"]'