WebKit

... but they've thrown away all the former builds! :-(
The link I mentioned in a comment below for OS 10.9 is (probably definitely) broken.

My home computer is a 2008 Mac Pro and I'm having problems with Safari in el Capitan and some sites where I get "Safari can’t open page" "Protocol Error NSPOSIXErrorDomain:100..." Does this help with those issue?

I have no idea. But basically, these are development builds, I've used them as an experimental option for resolving issues when Safari stopped being supported in an OS — but after a while, you're a lot safer using a browser that gets updates such as Firefox or any Chromium based browser (Opera, Vivaldi or Marmaduke's Chromium — a really good Chromium build including codecs! Vivaldi is great but has historical issues with videos on Mac. I really like Opera and its integrated ads filter and free VPN!)
Out of curiosity, can you give an URL example that triggers that error?

Hi Albabe!
I'm just seeing your post right now... ;-)
In the meantime I've updated my OS 10.9 to 10.11, but that site loads perfectly here in Safari 11.1.2 with no such error in the Console neither.
Are you sure you've got that version of Safari?
If yes, I would check relaunching it with extensions disabled, then in a clean user account (a Guest account is good).
If it's a user issue: trash ~/Library/Safari (keeping a copy of the bookmarks file at hand), ~/Library/Preferences/com.apple.Safari.plist and com.apple.Safari.Extensions.plist, reboot dans see...
If it's a global issue (and not present in Firefox or any other browser), I'd try reapplying the last 10.11.6 OS update, and/or the last security update.
But I've already encountered issues in Safari 11.1.2 on other sites, so unfortunately, I think we can consider it a second-hand browser as it probably won't be updated any more...

See my warning from early 2016 below. ;-)
MU doesn't seem keen on correcting this...
Unfortunately, there was a page where you could DL an old version clicking on its number, now you have to select the URL in that list: https://nightly.webkit.org/builds/trunk/mac/all and paste it.
It would be so nice if they made a page with the latest nightly for each OS, but no. :-/
BUT, we're quite lucky on 10.9 as the last build (r192684) is running nicely — which is not the case for the last 10.8 build (r174642) that I tried recently for a friend on his old Mac.

It would be nice if they had a few links, one for the most Current for each OS. They do it on other Clients, why not this?

Oops, my bad. I'm looking for a WebKit that works for us folks who use Mavericks and can't afford to update to Sierra because of the cost to replace the apps I use.

Albabe> I posted the direct link for the latest 10.9 build in my post here above. :-)