Remove need to readjust margins in 404.html

Since some of the users prefer to just modify the placeholder 404
page instead of replacing it with their own, this commit makes it
easier to do that by allowing them to modify the content without
worrying about readjusting the margins.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Note: The different in size compared to the old 404 is negligible:

 * old 404:

   original size:         1242 B
   gzipped size:           574 B

 * new 404:

   original size:         1274 B
   gzipped size:           570 B

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Ref: h5bp/html5-boilerplate#1567.
Close: h5bp/html5-boilerplate#1567.

404.html

@@ -7,22 +7,23 @@
     <style>
 
         * {
-            line-height: 1.5;
+            line-height: 1.2;
             margin: 0;
         }
 
         html {
             color: #888;
+            display: table;
             font-family: sans-serif;
+            height: 100%;
             text-align: center;
+            width: 100%;
         }
 
         body {
-            left: 50%;
-            margin: -43px 0 0 -150px;
-            position: absolute;
-            top: 50%;
-            width: 300px;
+            display: table-cell;
+            vertical-align: middle;
+            margin: 2em auto;
         }
 
         h1 {
@@ -32,19 +33,19 @@
         }
 
         p {
-            line-height: 1.2;
+            margin: 0 auto;
+            width: 280px;
         }
 
-        @media only screen and (max-width: 270px) {
+        @media only screen and (max-width: 280px) {
 
-            body {
-                margin: 10px auto;
-                position: static;
+            body, p {
                 width: 95%;
             }
 
             h1 {
                 font-size: 1.5em;
+                margin: 0 0 0.3em 0;
             }
 
         }

CHANGELOG.md

@@ -1,5 +1,7 @@
 ### HEAD
 
+* Remove need to readjust margins in `404.html`
+  ([#1567](https://github.com/h5bp/html5-boilerplate/pull/1567)).
 * Update to Apache Server Configs 2.7.0.
 * Add `/.editorconfig` file
   ([#1561](https://github.com/h5bp/html5-boilerplate/issues/1561),