HTML5 is not video and does not make video better. HTML5 has a video element, yes, but browsers still need to support that element and the programmer/site operator still needs to encode the video in multiple formats. Will this change? Probably. I’m looking forward to the day when I can upload my video file in whatever format and have it play across all browsers, like magic. But that day is not today. Today we encode in multiple formats, sometimes use the video tag and sometimes use the object tag. We’ll get there but it’s not now.
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HTML5 is not more secure than HTML4, stating such is nonsense, it’s a markup language. Are browsers better, arguably more secure? Yes. But those same browsers are better at rendering HTML4 too and their security applies equally to all versions of HTML5. It’s actually plausible to say that HTML5 and a browser’s implementation thereof makes users less secure due to as yet undiscovered exploits in the way those browsers interpret the new specification. Think: Web Storage.