IsomPreviewApplet is an applet that is present from Minecraft Infdev to early Minecraft releases this applet is used to view a world througn an isometric view hence the name.

Backstory of IsomPreviewApplet

IsomPreviewApplet was first introduced in Minecraft Infdev 20100616-2 1, The IsomPreviewApplet was only supposed to be implemented in the Minecraft website at the time, But, it was removed and it was just left in the code unused.

Applets at the time was only accessible through websites, html, and etc. But, You can launch it by creating a launch wrapper which the main Minecraft client also uses, here is the code to be able to launch IsomPreviewApplet.