elm-pages-v3-beta/docs/FAQ.md

## Can you pass flags in to your `elm-pages` app?
I'm trying to figure out the most intuitive way to model the concept of flags in `elm-pages`. Because the value of flags will be different during Pre-Rendering and Client-Side Rendering, just passing a single flag value would be misleading and make it seem like you have access to JS in the context of the user's browser on init. But you have to account for the Pre-Rendering phase as well, so flags has two different meanings.

So for example, if you get the window dimensions from the flags and do responsive design based on that, then you'll see a flash after the client-side code takes over since it will run with a different value for flags. So that semantics of the flags are not quite intuitive there. You can achieve the same thing with a port, but the semantics are a little more obvious there because you now have to explicitly say how to handle the case where you don't have access to flags.

The discussion is being tracked here: https://github.com/dillonkearns/elm-pages/issues/9.

I think the likely solution here will be to pass in flags, but wrap it in a custom type that makes the current lifecycle stage more clear:

```elm
type Flags value =
  FromPrerenderer value | FromUsersBrowser value
```

Right now, you can achieve the same result with a port.

## How do you handle responsive layouts when you don't know the browser dimensions at build time?

A lot of users are building their `elm-pages` views with `elm-ui`, so this is a common question because
`elm-ui` is designed to do responsive layouts by storing the browser dimensions in the Model and
doing conditionals based on that state.

With `elm-pages`, and static sites in general, we are building pre-rendered HTML so we can serve it up
really quickly through a CDN, rather than serving it up with a traditional server framework. That means
that to have responsive pages that don't have a page flash, we need to use media queries to make our pages responsive.
That way, the view is the same no matter what the dimensions are, so it will pre-render and look right on whatever
device the user is on because the media queries will take care of making it responsive.

Since `elm-ui` isn't currently built with media queries in mind, it isn't a first-class experience to use them with
`elm-ui`. One workaround you can use is to define some responsive classes that simply show or hide an element based on
a media query, and apply those classes. For example, you could show the mobile or desktop version of the navbar
by having a `mobile-responsive` and `desktop-responsive` class and rendering one element with each respsective class.
But the media query will only show one at a time based on the dimensions.