I can see how some people would want a web-based tool to used quickly and easily, although personally it's not the approach I would take. Many years ago I wrote python programs set_minus, set_union, and set_intersect that take two (or more) sets and print the appropriate result. For example

    set_minus A B

... prints every line in A that's not in B. Similarly the others.

Fast, clean, command-line, results to be piped to be processed or put into files.

Alternatively, if you want to union files you can just do this:

    sort A B | sort -u

If you want to find the intersection:

    sort <( sort -u A ) <( sort -u B ) \
    | uniq -c \
    | grep " 2 " \
    | sed "s/^ *2 *//"

So all these things are easily done from the command line.

Even so, as I said, I can see how some people would want a web-based tool, so thanks for sharing!