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You can make this work by changing just one line in the source code, src/mod_tidy.c

In the current version, it is line 610...

Original Version:
ap_fputstrs(f->next, ctx->bb, ctx->htmlbuf.bp, NULL);

Changed Version:
ap_fputstrs(f->next, ctx->bb, tidyhtmlbuf.bp, NULL );

Rebuilding and re-installing with that change will make all the HTML output pretty-printed.