The video encode formats depend on the browser, and you can see which are supported in the About dialog of Construct. Note browsers support different encode formats to decode formats - for example Chrome can decode MP4 but only supports encoding WebM. The default "Auto" option is a good default as it will use a format that works, and if you manually change it to an option that is not supported it will not work. You can also try recording a WebM video and use external video tools to transcode it to MP4.