HELP pls: You have been redirected to the basic HTML version because this browser is not supported. To use standard view please upgrade to a supported browser.

Do you happen to be using the JS Blocker extension?

Because I ran into that issue two days ago and it was caused by the extension.

I tried to log into a few Gmail accounts and each time I got a page with this:

And every time I hit the "latest Gmail" button, the page just refreshed - I was caught in a loop.

Less than 30 minutes ago I tried again but clicked the HTML button and saw what you saw:

I disabled JS Blocker and was able to switch to standard Gmail at the bottom of the page:

JS Blocker says nothing is being blocked in Gmail, but obviously something is.

What are the details? What web browser and what website? Certainly there are browsers where the versions for Yosemite haven't been updated for the latest security patches. Most are recommending using at least El Capitan because of security concerns.

It's up to the webmaster to lock out browsers that they consider inadequate. I see that a lot with banking websites.

Apple *has* put out an updated version Safari—you haven’t installed that update on your computer.

You must update your macOS software to get the updated Safari version.

If you can’t/won’t do that, you can add your Gmail account(s) to the Mac Mail app, or use a different browser—but if you are running Yosemite, as your info indicates, even Firefox or Chrome might be outdated. Google would determine that, though, since they are responsible for making sure their services, like Gmail, are compatible with Apple’s stuff, not the other way around.