In that context, you aren't using strings. You are using bytes. HTML without interpreting it as strings isn't really HTML, nor is it a string. It's just a blob that is passing through.
This is false more often than not. Many programs taking user input will treat it as string, assuming specific encoding or compatibility with screen output/some api, at least in some code paths. For example if you print an error message when you can't open some file, you are very likely to assume its encoded in a way terminal can handle, so its no longer "just binary data".
Yes, I have to worry about how to make a "best effort" to show it to users, but in all internal code paths it must stay as "just binary data", else I lose information. This is exactly how chrome and Firefox handle headers internally.
