I do not have experience with pure Flash but with Flex4 that is a powerful GUI for applications. I can share here what I miss from those days:
- you had types, performance was important so you had generic types like Vector , the code was compiled and it was really fast
- there were built-in efficient GUI widgets, you could have a list or table connected to a data provider with tousends of items but the components were smart and only created enough UI widgets as needed to render, where in HTML you either have to create a LI element for all your 10000 elements, create pagination(is not built in), create infinite scrolling(not built-in)
- it was super easy to customize or extend the provided things if you needed , as an example in HTML look at the it is not customizable, everyone has to implement it's own using divs, css and js , but everyone gets it wrong in the corner case(you have issues where the dropdown pops outside the screen, missing events, hell even youtube search box sometimes breaks and the dropdown popup remains stuck open, so even Google engineerscan't create a working dropdown component
- performance, remember when a blinking cursor was causing an electron editor to use a lot of resources , the issue with web is that was designed in rush for showing documents.
- layouting components was super easy , theming was easy - with css in large old projects there are so many issues( I know people use less but this is not standard )
I have some solutions too, browsers need to provide more powerful components, if we want to do RIAs or desktop apps then we need the same components that desktop toolkits offer with the same customization. Other issue I seen was in a project where the devs ahd to import a third party library because the scrollbars can't be themed in all browsers , I am not refering to the right side of the browser ones(I know sme people hate if you change those) but scrollbars on a list or dropdown component. So you have projects where you import 1 lib for scrollbars, 1 lib for modals, 1 for fancy buttons and inputs, 1 for drag and drop, 1 for calendar widgets, 1 for dropdown that allows you to have icon + text, and of course a framework like angular or react . Browsers should check what developers need the most and add that as standard( ex better dropdowns) you would still have the ability to make your own shit and use your preferred framework