Fixed Header Design Pattern:

In a Fix

When you pin your header to the top using position:fixed it's no longer contained by your page-wrap. So I start the .page-wrap after the header, and duplicate the width and margins on a wrapper div within the header:

/* match backgrounds */
body, .header-top { background: #eaeaea; }

/* can't assume only one "header" in HTML5 */
.header-top { 
  /* pin to top - 0 is default */
  position: fixed;
  /* raise z-index to cover */
  z-index: 1;
  /* cover full width when zoomed */
  width: 100% }

/* match widths, margins & padding */
.page-wrap, .header-wrap { 
  width: 90%; 
  min-width: 320px; 
  max-width: 640px; 
  /* center on page */
  margin: 0 auto; 
  /* separate content from window edge -- 
     here (not on body) to work with fixed header */
  padding: 0 10px;}

I also add a little padding between the window edge and the content, for when the window is at or below the min-width specified. (Yes, I'm just that anal.)

Mobile: Your Call

Mobile Safari supports position:fixed ( with some quirks ), but that doesn't mean it's a good idea. Make sure to test your fixed header on mobile, in both orientations:

Consider using CSS media queries to target device-width or orientation and then change the header position from fixed to absolute, so that it scrolls — or make the fixed header more compact. You could also change the initial-scale:

<meta name="viewport" content="width=device-width, initial-scale=.5">

Scaling on iOS has its own idiosyncracies: Here, the fixed header tries to maintain its percentage-based width and reflow, whereas the scrolling content zooms off the screen and can be moved around freely (unless user-scalable=false). That's why I set the header at 100% with a background, so it always covers.

If you know the content will be scaled, you might choose to have your page-wrap and header-wrap set to 100% instead of 80 or 90 — or change it at smaller sizes with a media query, so that the header stretches to the edges.

It's become common practice to include <meta name="viewport" content="width=device-width"> in the document head, but make sure that gives you the result you want. It's powerful meta mojo that you may not need if your site isn't fully-responsive.

Hitting the Heights

In this example, I've specified a height for the header, knowing its content. I can decide how it will reflow when the window resizes, using media queries or absolute positioning (here, I have it expand up from the bottom).

But what if the header content is unknown or dynamic? One idea is to replace the top padding on the page-wrap with a duplicate of the header content and its styling (minus the position:fixed). The fixed header simply covers it — though I go ahead and set the duplicate to be invisible and ignored by screen readers. ( Example CodePen here — right-click to Open Link in New Tab.)

A last note: Remember that your #top anchor can't be in the fixed header.