There is almost always a Sierpiński Triangle in isotropic 2D CA:

`x = 5, y = 5, rule = 24/2458/5`

2.2A$2.ABA$2A2B$ABADB$.A.DC!

`x = 2, y = 2, rule = 256/256/256`

2A$A!

`x = 1, y = 1, rule = /1245678/256`

A!

`x = 1, y = 1, rule = B1/S0V`

o!

That last one may not seem like it has a Sierpiński Triangle in it, but it is actually simulating Wolfram 22 on its axis and Wolfram 90 on its edges, and they look like this when plotted:

(W22)

`x = 1, y = 1, rule = W22`

o!

(W90)

`x = 1, y = 1, rule = W90`

o!

B1245678/S/∞ constructs an approximation of the Sierpiński Pyramid if you make the 3rd dimension the cell state.

Sorry, some of my rules before April 20 2019 have Unicode characters that are not compatible with Golly; you will have to remove them when pasting them in your text editor.