Deprecated in HTML5. Do not use.

<app> HTML Tag

Element of
Applets
What does <app> HTML Tag do?
The <app> element was a predecessor of the <applet> element which was deprecated in HTML 4.01 and removed from the specification entirely in HTML5. Modern equivalents include <object> and <embed>.
Display
block

The <app> element was renamed to <applet>, but has since been deprecated in HTML5. You may need to use one of the following instead:

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