The wide variety of image formats and the lack of support for them in early versions of browsers led to their designers building in a way of adding more formats through extensions, or plug-ins. These added capabilities to the browser without the need for building them into the software itself. Innovative companies soon exploited this, and created their own formats for all kinds of media, not just images. Though plug-ins enabled the browser to view a wider range of material, they allowed an explosion of non-standard, proprietary formats, and they are not always available for all computing platforms (e.g. Macintosh, UNIX or Linux, etc.).

Plug-ins are now most often used for “cutting edge” formats which are still being developed or agreed upon; SVG was in this stage of development but is now built into most browsers.