Accordig to filext, nothing. It’s obviously an HTML file and the browser is reading it as such. Why the webmaster chose that extension is unclear. Maybe it makes more sense in Norwegian.
Note that if you are judging it to be HTML by looking at the source in your browser, that isn’t really valid. All that’s telling you is that the result of processing that URL is HTML, not anything about the content of a file on their server with an “.ece” extension, which could be some sort of scripting language for generating the content, handled through a plugin or CGI gateway on their server. Just like looking at this page source doesn’t tell you much about what “showthread.php” contains. All it gives you is a sample of what that .php script can generate. That said, I don’t recognize “.ece” either.
It’s also possible that it really IS actually an HTML file on their server, and they are following some private convention for organizing their files which involves some invented extensions that make sense to them. If it isn’t recognized as a plugin of some sort, your web server will just serve up the file contents regardless of extension.