An MP4 file is a multimedia file commonly used to store a movie or video clip. It may also contain subtitles or images. MP4 is short for MPEG-4 Part 14, which is a container format based on the QuickTime File Format (QTFF) used by .MOV and .QT files.

HTML5 has animation and responsive/adaptive design capabilities among other features and these require CSS and JavaScript. The CSS and Javascript can be included in the .html file or can be sourced externally using .css and .js files (just like images are called externally).

Because every external call from the .html file slows download times, it can be better to include that code within the .html file.

A moving picture in GIF format, which comprises a series of frames. When displayed, they provide an animated sequence that cycles over and over without stopping. Although popular on the Web, animated GIFs are larger than single-frame GIFs and take longer to download. Typically used for ad banners, the animation is usually confined to small areas of the image to keep the file size short.