What is applet?? Support in html??

Answer 1

Applet is an embeded Java application in webpage via JNLP (Java Network Launch Protocol).

However it's going obsolete and browser vendors plan to drop its support as the plugin is too buggy to maintain and open a lot of vulnerabilities.

Answer 2

Well it WAS supported in HTML4
But they decided it was too umsafe to be there so in HTML5, the latest version of HTML, they decided to remove it