They know that anything simple and widely compatible is against their goals. Their excuse is "web standards are open" --- the same highly complex standards they control and churn so frequently that implementations beyond their own are basically discouraged and can't keep up. Their hostilities have ramped up most recently with things like WEI, but this is their next step. It was never really about making things better for the users. It was always about complete control: taking over the Internet and forcing users to a run a "supported" software and hardware stack, one that they both fully control and will deprecate or "EOL" on their schedule to force users into the next stage of their control.

Basic HTML, like any other simple and easily understood systems that anyone can implement without much effort, are essentially a threat to their agenda.

To paraphrase an old saying: "Gmail ain't done 'til Firefox won't run."