This page is for testing interactions between predictive text and keyboard rules + rule groupings where it is possible for context to be altered without the final group having a matching rule. See PR #6473 for additional details.
About the "Unmatched Vowel Rule" test keyboard: this keyboard will only allow up to two vowels in a row. It will not emit any further vowels until the cluster is broken up, be it by consonant or whitespace. Note that ALL vowels that it emits are output as a result of "default" behavior, not a keyboard rule!
Also, while it tends to block vowels, it instead duplicates the English semi-vowel. It'll automatically output `yy` when `y` is typed and `YY` when `Y` is typed.