The size of the viewport is too small. Is there or will there be a way to fullscreen Decker in the browser?

Web-decker is designed to find the largest integral upscale of the deck size (by default, 512x342 pixels) that will fit in the containing window or iframe, and letterbox out the remainder. Results will vary based on container dimensions and aspect ratio.

Stretching to fit the container is possible, but would result in blurred pixels. (Even as it is, getting some browsers to display pixels accurately is remarkably challenging.) If you want to tweak how this sizing works, take a look at the "resize" function near the bottom of the file in your web export.

Another variable to play with is the "size" property of the deck itself, which can be manually edited in a .deck or .html file. Making it much smaller than the default can result in bits of the UI getting squished or inaccessible, and making it significantly bigger could reduce performance and make decks harder to view on devices with small screens. The hard minimum Decker will allow is 320x240.