With Cloudflare's Mailchannels integration that came out yesterday, it's now possible to send emails for (practically) zero cost from workers. I rolled out this example of how to set up a SvelteKit based worker that you can use to develop HTML based transactional emails locally as regular web routes, then use them on production by simply POSTing the subject, from, and to fields to the routes. The hook SSRs the route, validates the request, generates a text/plain version and sends the email.
Layouts work, so you can make reusable layouts, and use all the Svelte good stuff.
There's template variables using {{{triple_braces}}}, which get replaced from the POST request. This is to work around SSR not exposing query parameters for modules (from what I could see).