All of my projects are ActionScript only projects, because of this they can be used in any environment including within the Flex framework.
I've never used this project so I can't really say but the base of the project is tightly linked the the concepts of Flex. Everything is a UIComponent and it follows the normal Flex component life cycle in many spots. If you are working on a Flex only project maybe this is good for you but from what I have seen in this project it has few features that match the HTML and uses the Flex CSS I think.
In my opinion the Flex framework is slow and bloated. The CSS support is extremely basic and follows it's own conventions rather than anything to do with web standards. If you want to be able to parse some random page following as much of the standards as you can should be the goal.
I've written a HTML/CSS libraries. I wouldn't call any of them perfect but they all have there place. I started with
http://code.google.com/p/htmlwrapper/ which was the first AS3 one as far as I know, it was trying to act as a plugin on an HTML page, the goal was to render anything in the source within flash instead of by the browser, I was trying to solve the problems with browser incompatibilities and add features like animation and filters to the browser long before HTML5 and CSS could do these things. But it didn't have an API and many of the tags you might want are not implemented.
I then got a job and created a closed source one that had a way to script things and add for loops to html code, the markup turned into a dynamic scripting language. While doing this I learned a few things and saw how people wanted to use it.
I then tried to make the simplest HTML/CSS parser I could.
https://github.com/talltyler/MiniHTML it's extremely fast and does 90% of want anyone might need but it needs XHTML and all of the properties are named based on the Flash equivalent not the standard HTML or CSS names (like x,y instead of top,left). If you are coding custom pages this doesn't matter but if you are rendering random pages it wouldn't work at all.
I then wanted to make something better, something with an API, something that was more than just able to display HTML/CSS, I wanted it to have plugins to render anything and take in any type of data. I want to be able to extend HTML and to control what was rendered like how JavaScript can manipulate a page after it is rendered. And if you can render one web standard the utilities that you are writing are the same to render any other web standard. So with ASTRID is mostly these utilities to break down development of any of these web standards and really utilities to help with development of any ActionScript project. The HTML and CSS part of it also got a lot more love and includes many of the new HTML5 features and many of the things that people requested from my HTMLWrapper projects. I never fully implemented tables but or a few things like z-index and frames but all of this is started and almost works.
Maybe ASTRID isn't for everyone or every project. If you want something simpler or specifically focusing on HTML/CSS maybe one of the other projects I built would be better. But without all of this larger structure ASTRID would not be able to do all of the things that it can do.
Also my projects and this one you have linked to are not the only HTML renderers. There are maybe 4 or 5 other ones, I don't have links for all of them. If you have time, try them all out and see what fits best for your project. Let me know what you end up going with, even if it's not my stuff.
-Tyler