Perhaps hstspreloading could be kept, but not enforced by default, and a TRACE or DEBUG message emitted when a host is found on the list and doesn't match the preferred scheme?

So, it's a wooly issue, but I think yes let's get this in and treat HSTS as appropriate as a browser feature, but not necessarily desirable in client library.

I would hate for the great work in the hstspreload module to go unused. Would it be good to include a brief mention in the docs for those that do want that functionality?

I'm trying to think of how people could enable this without us having to support it in core with a flag…

I don't think a custom transport would be the way to go, since transports don't deal with client logic like "optionally modify the request URL".

Probably more like a client subclass, then…?

import httpx
import hstspreload

class HSTSPreloadMixin:
    def build_request(self, *args, **kwargs):
        request = super().build_request(*args, **kwargs)

        url = request.url

        if (
            url.scheme == "http"
            and hstspreload.in_hsts_preload(url.host)
            and len(url.host.split(".")) > 1
        ):
            port = None if url.port == 80 else url.port
            request.url = url.copy_with(scheme="https", port=port)

        return request

class AsyncClient(HSTSPreloadMixin, httpx.AsyncClient):
    pass

class Client(HSTSPreloadMixin, httpx.Client):
    pass

Incidentally I think this could also fit in a "middleware" kind of concept (#345, also mentioned in #984), but that's definitely not something we'll have 1.0.

Possibly, but I was thinking of and even simpler approach/demonstration:

util.py

from typing import Union

from httpx import URL
from hstspreload import in_hsts_preload

def check_hsts(url: Union[str, URL]):
    if isinstance(url, str):
        url = URL(url)
    if in_hsts_preload(url.host):
        return url.copy_with(scheme="https")
    return url

...
main.py

import httpx

from util import check_hsts

httpx.get(check_hsts(the_url))

or with base_url:

with httpx.Client(base_url=check_hsts(the_url)) as client:
    client.get(path)