Sierpinski Gasket by Trema Removal
As of 2018, Java plugins are not supported by any browsers (find out more). This Wolfram Demonstration, Sierpinski Sieve, shows an item of the same or similar topic, but is different from the original Java applet, named 'Tremas'. The originally given instructions may no longer correspond precisely.
(image below from deprecated 'Tremas' applet)
Keep clicking in the applet area