The ascii art is broken.
One day I'll write a Pandoc filter that pipes these diagrams through a Shaky or Ditaa type tool first…
Looks weird for me on Mac desktop Chrome. It looks like the default monospace font, Courier, lacks box drawing characters. So it probably falls back to some other monospace font which likely has different metrics.
A good CSS font stack for monospace fonts is something like:
font-family: Consolas, Monaco, monospace;