App store update page doesn't seem to be loading properly on Mac OS El Capitan 10.11.6

Solution 1:

It's been that way for quite a while - I'd assume it's simply no longer supported for El Cap, but I have no citation for that.

You have to bear in mind that there will no longer be any updates for any apps on El Capitan [nor indeed for the OS itself] - many apps are now minimum spec Mojave, some a little older some newer, but no-one is going to be testing against El Capitan any more. It was superseded 5 years ago & generally support is to 3 years.

As noted, this applies up to High Sierra - the only difference on High Sierra is it lists all the updates you can't have, also in plain text ;)

Solution 2:

This is a known problem. On all osx versions prior to Mojave Appstore is displayed in "text" mode. CSS issue not resolved to date. This does not, it seems, interfere with proper operation.