WKWebView

WebView (on Mac) is the old WebKit view that runs entirely in-process. WKWebView is the new hotness that runs in its own process, for performance and security (so they claim). It has a number of downsides, though, most significantly that it ignores custom URLProtocols. It'll load the protocol and claim to support it, but then ignore it completely.

• UPDATE: In 10.13, there's a WKURLSchemeHandler that seems to resolve this! I haven't tried it yet.

Other downsides of WKWebView:

• Xcode 8 can't add it in IB (but I hear Xcode 9 can?)
• You get a "Reload" context menu (or "Open in New Window", etc?), and it's not easy to disable
• Printing is broken
• Even more?