how it is possible from a technical/coding point-of-view that a text-based markup language (HTML5) could be considered a replacement to a multimedia framework (Flash)?
HTML5 is the new hot name for "web technologies stack", not just a "markup language".
It has a programming language (JavaScript), like Flash does; a set of APIs, significantly expanding as part of the efforts commonly labeled as "HTML5", ways to draw graphics (SVG,
Unlike Flash, the "HTML5 applications" are processed by the browser, not by a proprietary browser plugin, which is good - at least for browser vendors (see the relevant piece of this blog post - thanks Jotham). For example, it makes possible for a browser vendor to fix issues (and not wait for Adobe) -- IIRC Apple cited Flash as being #1 reason for crashes among all crashes on the Mac.
[edited to remove "open standards are better" claims which I am too lazy to back up]
