You're using a markup language to define the user interface - why? If the point was to make it look more familiar to web developers then why not just let them use HTML (as in, HTML separated in it's own document)? I know, the idea of Opa is to be one language to develop the whole stack, but what you get here is an incosistent look of your language. If you're going to let the developers define GUI along with the rest of your logic, it would be much better if it was done in a consistent way.

On the other hand, one could argue that an XML-like language is better for GUI development because it lends itself naturally to hierarchical structures (which a GUI certainly is). And I can actually agree with this. But in that case, I would definitely keep it separated from the rest of the code. Maybe this is just me, but I prefer the concept of separation of presentation and logic.

Usually, one of the best things a web company can do is to not make their developers wear the hat of a web designer, but instead hire an actual web designer who knows her job and who can create a great GUI. But I get the feeling that this person would be very confused if she had to work with Opa. Let the designers work with what they know best, HTML and CSS, and let the developers work on creating a logic behind this presentation layer.

So, I hope this comment clarified a bit what I meant when I said that Opa looks crowded. Of course, these are only my personal views and preferences and are meant as constructive criticism.