> In HTML you should be able to use <q> – I know that doesn’t work reliably
in browsers (some add straight quotes to my CSS-configured guillemets).

The Converter class maps token replacements:

https://github.com/DaveJarvis/keenquotes/blob/d6c9761f8fe1ae96391f25dc73be52050a148e37/src/main/java/com/whitemagicsoftware/keenquotes/Converter.java#L15

It'd be trivial to use <q> and </q>, instead. For my purposes, HTML
entities work.

> Using \quotation / \quote I avoid typing quotation marks in most cases.

When writing plain text documents, adding TeX code or HTML code to
prescribe how the document should be presented is best avoided, so as to
keep the document decoupled from a particular tool chain. YMMV. A deeper
solution allows users to type the correctly curled quotes directly into the
document.