Well they could have a list of the properties without them all rendered and then render just the property they need when someone clicks on the property name.
And why should it render all examples on one page?
They could have a static page for every property so one could link to http://cssreference.io/align-content
instead of http://cssreference.io/#align-content
and get just that static page. That should be pretty quick, especially with long caching of all other resources than the actual page content.
There's lots of different ways to have done it without having to do it all at once.
Then someone would have complained that this site needed JavaScript to handle those click event.
Or, would have complained that it was stupid to actually use an anchor tag to link to god forbid another complete page load.
Or, if they used a javascript framework to make deep links someone would have complained that they used a javascript framework for a static page, or even maybe they used the wrong JavaScript framework.
I think it's much better UX to show everything on one page in this case. You're speaking like this is an application, completely disregarding the purpose.
The best solution would be to lazy load. If you can only see the first 3 property on page load, there is no gain in having them all load at the same time.
Disable all the styles. Enable the styles just for the sections that are currently visible.
It would require setting section heights to fixed values so you can know which section is visible and enable and disable styles for them without changing the sites height.
Except that it loads slowly which hurts the usability so it's not really that I prefer a certain layout but more that I prefer the page to load easily.
And why should it render all examples on one page?
They could have a static page for every property so one could link to http://cssreference.io/align-content instead of http://cssreference.io/#align-content and get just that static page. That should be pretty quick, especially with long caching of all other resources than the actual page content.
There's lots of different ways to have done it without having to do it all at once.