Any idea why Safari (11.1.2) for macOS High Sierra 10.13.6 is not rendering this web-page correctly?
On my machine (iMac late 2015, macOS 10.13.6, Safari 11.1.2), the cause of the bad rendering was the AdBlock extension. I removed it, restarted Safari, and Safari rendered correctly. I then re-downloaded AdBlock, and Safari continues to render correctly.
I moved too fast doing this, and didn't note the version number of the AdBlock I removed, so don't know if the not-working AdBlock I removed was the currently-downloading version. In any event, the just-downloaded version doesn't interfere with Safari's rendering.
Enable Develop Menu in Safari by going to
Safari → Preferences → Advanced.
Now hit Option + Command + r to reload the web-page from origin.
The CSS for the page is missing/unreachable thereby rendering it this way.
If the problem persists, wait for some time and try again.
Additionally, you can also try clearing local cache by going to
Safari → Clear History....