What does that mean as related to 'competition in the browser space' ?
The assertion is that the major browser owners (Apple, Microsoft, Google, Mozilla) don't want competition, so they keep CSS complicated.
All four of these companies are members of the CSS Working Group. So the ability to do this is within their power.
You control the browser, you control the ad experience, the use data, etc.
It's unclear to me what an App Store has to do with anything. (Microsoft does have an app store, by the way. https://apps.microsoft.com/home?hl=en-US&gl=US )