position:fixed + margin:auto;

dabblet.css

/* position:fixed + margin:auto; */

html {
background: #f06;
background:repeating-linear-gradient(45deg, #acf 0,transparent 14px),repeating-linear-gradient(-45deg, #acf 0,#159 14px);
height: 100%;
}
body {width:800px;min-height:200%;margin:auto;box-shadow:0 0 5px;
background:repeating-linear-gradient(0deg, white 0,#eee 1em)}
.fixed {position:fixed;left:0;right:0;top:75%;}
.fixed p {width:800px;margin:auto;position:relative;}
.fixed p a {position:absolute;
left:800px;width:2em;line-height:2em;text-align:center;background:repeating-linear-gradient(0deg, white 0,#eee 1em);border-radius:0 1em 1em 0;box-shadow: 2px 0 2px black;

dabblet.html

<div class="fixed"><p><a href="#">top</a></p></div>

dabblet.js

// alert('Hello world!');

settings.json

{"view":"split","fontsize":"100","seethrough":"","prefixfree":"1","page":"all"}