Good to know! I’ve been using local testing and fortunately haven’t run into a case where the tests pass local and fail on their servers. Yet.
My most intense solution so far had been a very multi-core Knapsack solution. The tests they provided are pretty minuscule, which probably helped.
There are a whole lot of “templating” libraries which do what you’re asking for. I have used Hiccup for Clojure and Giraffe for F# successfully, and you can probably find others for languages you already know.