I'm working on a project to preview MXML live. It would probably take about a week or two to put it online. Then you could type in MXML and see it live which you can do currently with the downloadable desktop version. There's another project that's it's based on MXML called Radiate that let's you create a design and see the MXML created generated live. You can then copy and paste that MXML into your Flex project or publish it online.
FYI and disclaimer: I've been working on it since around 2010 and open sourced it a few years ago. Radiate is based on the open source project Radii8 at Apache Flex so if it's something you'd like to improve you can download the sources. There is a commercial version that I sell for the sole purpose of having money to keep developing it but if you can't afford it or just want to try it first you can download it at the links above. Another disclaimer: since I love the work zwetan is doing for the community and RedTamarin (server side AS3) I would like to donate to his work from sales. But I have to buy my yacht first. After that proceeds will go to his ramen noodles.
FYI My goal is the similar to his: Grow the ActionScript 3 community and promote the AS3, Flash, and AIR platform and ecosystem. Which has made development and creativity in the 21st century a pleasure rather than a pain. Which makes possible new and unique apps, web sites, art, music and many more things possible.
Also, to contribute more to the conversation, there's a project called Moonshine IDE that is an AIR app meaning it will run on both Windows and Mac. l looked into it early on when I was developing Radii8/Radiate but it was abandoned a few years ago. Recently, someone said it had started back up. Not sure how far along it is but that might solve some issues. It's not clear what their goals are but it is great to see new projects for Flash and Flex development (really for web, mobile and application development).