Dido is an application (and application development environment) in a web page. It is a single web page containing rich structured data, an AJAXy interactive visualizer/editor for that data, and a “metaeditor ” for WYSIWYG editing of the visualizer/editor. Historically, users have been limited to the data schemas, visualizations, and interactions offered by a small number of heavyweight applications. In contrast, Dido encourages and enables the end user to edit (not code) in his or her web browser a distinct ephemeral interaction “wrapper ” for each data collection that is specifically suited to its intended use. Dido’s active document metaphor has been explored before but we show how, given today’s web infrastructure, it can be deployed in a small self-contained HTML document without touching a web client or server. ACM Classification: H5.2 [Information interfaces and presentation]: