Previous Attempts at Smarter HTML Browsers

Plug-ins Netscape made plug-ins popular, even including some with its browser, but the API, necessary markup, and unreliability of the model made it difficult to use across the Internet.
Applets Java seemed like the answer to this problem, until it became clear that applets were difficult to create, limited by both the technology and the security sandbox, and not well-loved by users.
Dynamic HTML Incompatible implementations (even today) and the continuing limitations of HTML make Dynamic HTML useful in only a limited number of relatively simple problem spaces.
Flash Loved and reviled by different parts of the Web community, Flash remains a Macromedia project devoted to constructing a certain kind of interactive document.