Visit this page in Safari: https://www.construct.net In Chrome and Firefox, it looks correct. In Safari only it has serious display glitches and different layering of elements. There appears to be some web compatibility issues with Safari here.
Looks like the "slantedBackground" <div> covers the rest of the content (maybe some z-index, composition type of a bug?)
This is a 3D transforms flattening issue.
<rdar://problem/83358264>