reframe

Reframe is a browser-based tool for adding a polished frame around a video clip and saving the result as an MP4 file. It was built specifically for dressing up phone screen recordings — placing the video on a square canvas with styling options such as an inset, rounded corners, a border, a drop shadow, and a custom background color or image.

You use it by dragging your video file onto the canvas, optionally dragging in a background image, adjusting the framing controls in a side panel, and clicking Export MP4. Audio is not preserved in the output.