We were on 69 for both of these platforms which is fairly old. This update also requires a temporary patch to the @bazel/karma npm package to disable chrome sandboxing on OSX as it is broken under Bazel as of chromium 73. Windows is still on Chromium 66 but updating this will require upstream changes to rules_webtesting as the archive name & executable name has changed as of 72 for Windows and hard-coded paths in rules_webtesting break things.
PR Close#30502
Greg Magolan