These indicate the level of support for the given item in each engine. By default, the most recent version of the engine is implied. However, a specific version number can be listed; when this indicates full support, it's the initial version of the engine fully supporting the item.
^ abcdKHTML did not have any native image support. This was by design: in Konqueror, multimedia files were handled by KParts.
^JPEG 2000 support was only available on the macOS platform prior to the release of 10.15 Catalina, and required QuickTime 7, since Safari 12 dropped official support for NPAPI plugins (including QuickTime). (Note that the referenced link leads to the Mozilla Bugzilla, where this was brought up and confirmed.)
^MSHTML only displayed progressive JPEGs after they were completely downloaded.
^There are 15 combinations of the color modes in PNG but not all modes are available in every image editor.
^Webkit incorrectly applies gamma correction to unlabeled PNGs.
^Presto supported gamma correction for all color profiles except for ICC-profile.
^In MSHTML, interlaced PNGs were displayed progressively, but in 1D instead of 2D.