tl;dr If you try to deploy the 'hello, world!' Eleventy site to Render, it will fail with a heifsave: Unsupported compression (via Template render error) error. To fix this, you need to remove the avif image type from .eleventy.config.images.js

slightly longer; might read Handling AVIF image types requires HEIC libraries to be installed on the system that builds the eleventy site, and render.com doesn't have this. Hence the error, and hence the need to remove this as a valid image type in the file that controls image processing.

even longer; not strictly relevant While render.com is a nice brand name, it doesn't make for very good search engine optimisation because the results are inevitably polluted by references to 'render' as in the verb 'to convert something'. It doesn't make finding problem resolutions any easier...