https://en.wikipedia.org/wiki/Hausdorff_dimension#Behaviour_under_unions_and_products Wikipedia page says that if $ \underset{i \in I}{\cup} X_i = X$ and $I$ is countable then $dim_{Haus}(X) = \underset {i \in I}{sup}\{ dim_{Haus}(X_i)\}$
But isn't Sierpinski triangle union of countably many one dimensional triangles? What am I missing?