What does it mean when someone like Scribd 'bets the company on HTML5' http://techcrunch.com/2010/05/05/scribd-html5/?
Or when Slideshare 'ditches Flash for HTML5' http://engineering.slideshare.net/2011/09/slideshare-ditches-flash-for-html5/ ?
Can someone explain what on earth this even means? If I have a site that doesn't use Flash, do I still need to care about HTML5? Why?
HTML5 allows a web developer to do some things that previously were difficult or impossible in plain HTML. As such, many sites who wanted to do those things used other plug-in technologies such as Adobe Flash to accomplish their design goals.
Moving to HTML5 has no strict meaning, but it generally means that they are moving away from non-HTML technologies like Adobe Flash and using HTML5 capabilities in modern browsers instead to accomplish their site designs.
One big challenge when moving to HTML5 (besides doing a whole new site design) is how to handle browsers (like IE) that do not support the relevant HTML5 technologies. For all it's foibles, one advantage of Flash was that a specific version of Flash brought a common feature set to any browser that it was hosted in. Since HTML5 comes with the browser, you get what the browser supports. There are, of course, extensive Javascript libraries to try to backfill some missing functionality in older browsers, but that is extra work and not a full replacement.