What do you use to render HTML
We use a custom rendering farm built around headless Chrome. We created our back-end to scale to meet any demand. Due to the nature of Chrome and how unstable the workloads can be our back-end system is designed to handle random crashes, and failures and automatically retry in those events. While failed jobs do cost us money to process, and take up valuable resources, we do not currently charge anything for failed jobs.
We also layered several additional technologies on-top of headless Chrome to allow a deeper customization, and richer experience. All of this is integrated together into a custom solution, which allows you to have more flexibility and control over how your documents are created.