Universal access is at the heart of the World Wide Web. It’s also something I value when I’m building anything on the web. Whatever I’m building, I want you to be able to visit using whatever browser or device that you choose.

Just to be clear, that doesn’t mean that you’re going to have the same experience in an old browser as you are in the latest version of Firefox or Chrome. Far from it. Not only is that not feasible, I don’t believe it’s desirable either. But if you’re using an old browser, while you might not get to enjoy the newest CSS or JavaScript, you should still be able to access a website.

Applying the principle of progressive enhancement makes this emminently doable. As long as I build in a layered way, everyone gets access to the barebones HTML, even if they can’t experience newer features. Crucially, as long as I’m doing some feature detection, those newer features don’t harm older browsers.

But there’s one area where maintaining backward compatibility might well have an adverse effect on modern browsers: security.

I don’t just mean whether or not you’re serving sites over HTTPS. Even if you’re using TLS—Transport Layer Security—not all security is created equal.

Take a look at Mozilla’s very handy SSL Configuration Generator. You get to choose from three options:

1. Modern. Services with clients that support TLS 1.3 and don’t need backward compatibility.
2. Intermediate. General-purpose servers with a variety of clients, recommended for almost all systems.
3. Old. Compatible with a number of very old clients, and should be used only as a last resort.

Because I value universal access, I should really go for the “old” setting. That ensures my site is accessible all the way back to Android 2.3 and Safari 1. But if I do that, I will be supporting TLS 1.0. That’s not good. My site is potentially vulnerable.

Alright then, I’ll go for “intermediate”—that’s the recommended level anyway. Now I’m no longer providing TLS 1.0 support. But that means some older browsers can no longer access my site.

This is exactly the situation I found myself in with The Session. I had a score of A+ from SSL Labs. I was feeling downright smug. Then I got emails from actual users. One had picked up an old Samsung tablet second hand. Another was using an older version of Safari. Neither could access the site.

Sure enough, if you cut off TLS 1.0, you cut off Safari below version six.

Alright, then. Can’t they just upgrade? Well …no. Apple has tied Safari to OS X. If you can’t upgrade your operating system, you can’t upgrade your browser. So if you’re using OS X Mountain Lion, you’re stuck with an insecure version of Safari.

Fortunately, you can use a different browser. It’s possible to install, say, Firefox 37 which supports TLS 1.2.

On desktop, that is. If you’re using an older iPhone or iPad and you can’t upgrade to a recent version of iOS, you’re screwed.

This isn’t an edge case. This is exactly the kind of usage that iPads excel at: you got the device a few years back just to do some web browsing and not much else. It still seems to work fine, and you have no incentive to buy a brand new iPad. And nor should you have to.

In that situation, you’re stuck using an insecure browser.

As a site owner, I can either make security my top priority, which means you’ll no longer be able to access my site. Or I can provide you access, which makes my site less secure for everyone. (That’s what I’ve done on The Session and now my score is capped at B.)

What I can’t do is tell you to install a different browser, because you literally can’t. Sure, technically you can install something called Firefox from the App Store, or you can install something called Chrome. But neither have anything to do with their desktop counterparts. They’re differently skinned versions of Safari.

Apple refuses to allow browsers with any other rendering engine to be installed. Their reasoning?

Security.