Flash took off, went to the Moon, and came back in flames. Silverlight just tried to kill Flash and failed (other factors led to the death of Flash). There was a time when multimedia on the web was only possible using Flash and HTML/CSS/JS was just a way to embed advanced software in the browser. In a way, WebAssembly and WASM binary format will become an open-source equivalent of what Flash Player and the SWF files were.