Playground
Write a HoloML page on the left and see it drawn on the right. The checker reads it as you type and lists its problems with their lines and columns. Nothing you write leaves this page: the scene is drawn by HyperSpace 3D's viewer, on this same site, and only the models your page names are fetched.
The page
Tab moves on from the text. Ctrl+] and Ctrl+[ indent and outdent the lines chosen, and Ctrl+Enter draws the page now (⌘ in place of Ctrl on a Mac).
Problems
No problems.