"So what's in common with these two games? They're both originally mobile games." (Emphasis mine.)

The sad part is that once converted to HTML5 these games won't be able to run on many (most?) mobile devices. Current mobile browsers are too slow or non-standard to run HTML5 based games. The exception is iOS 5 powered devices (iPhone/iPad) since the browser now includes hardware acceleration for both HTML5 canvas and CSS3 animation transforms.

For more details, see Spaceport.io's recent HTML5 animation performance tests: http://spaceport.io/spaceport_perfmarks_report_2012_3.pdf