Erik,
To be able to use XPages in a browser, the installation of a JVM/ JRE is not required. XPages use default browser functionality and doesn't require any plugins by default. The Dojo JavaScript framework is used in that context to solve issues in the use of different browsers and browser versions.
Domino 8.5.3 uses version 1.6 of Dojo. I just checked the
release notes of Dojo and IE 6 is still supported, so you shouldn't have any large issues there.
Mark