Checks the markup for well-formedness with the local XML parser (errors come back with line and column), then re-serialises it pretty-printed or minified, or keeps your source. The preview renders a sanitised copy — scripts, event handlers, foreignObject, animation elements and remote references are stripped and each removal is listed — over a checkerboard or a background of your choice at the chosen zoom. The header comment summarises elements, ids (duplicates flagged), viewBox and size; the secondary pane holds a data URI with <img> and CSS snippets. Everything runs locally.