Leading whitespaces in codeblocks are removed if syntax highlighting is done. Seemingly regardless of the language used. It is common to want 6 spaces denoting input lines in APL. However, if I use APL syntax highlighting, these spaces are removed. Non-breaking spaces or additional newlines do not help.

Here is a test notebook rendered with nbviewer: https://nbviewer.org/gist/rikedyp/59230d88565c3459d88230b8f2e3c256

And source as a GitHub gist: https://gist.github.com/rikedyp/59230d88565c3459d88230b8f2e3c256