The new version of Readability is closed-source, slow (as no longer client-side) and altogether not as good as the old one. I found a working copy of the old one and adapted it for Conkeror; the old one’s code is Apache License, so this is fine.
interactive("readability_arc90",
"Readability is a simple tool that makes reading on the web more enjoyable by removing the clutter around what you are reading",
function readability_arc90(I) {
var document = I.window.buffers.current.document;
var readConvertLinksToFootnotes = false;
var readStyle = 'style-apertura';
var readSize = 'size-large';
var readMargin = 'margin-wide';
var _readability_readStyle = document.createElement('SCRIPT');
_readability_readStyle.text = 'var readStyle = \'' + readStyle + '\';';
document.getElementsByTagName('head')[0].appendChild(_readability_readStyle);
var _readability_readSize = document.createElement('SCRIPT');
_readability_readSize.text = 'var readSize = \'' + readSize + '\';';
document.getElementsByTagName('head')[0].appendChild(_readability_readSize);
var _readability_readMargin = document.createElement('SCRIPT');
_readability_readMargin.text = 'var readMargin = \'' + readMargin + '\';';
document.getElementsByTagName('head')[0].appendChild(_readability_readMargin);
var _readability_readConvertLinksToFootnotes = document.createElement('SCRIPT');
_readability_readConvertLinksToFootnotes.text = 'var readConvertLinksToFootnotes = ' + readConvertLinksToFootnotes + ';';
document.getElementsByTagName('head')[0].appendChild(_readability_readConvertLinksToFootnotes);
var _readability_script = document.createElement('script')
_readability_script.type='text/javascript'
_readability_script.src='http://sean.whitton.me/geek/old-readability.js?x='+(Math.random())
document.documentElement.appendChild(_readability_script)
var _readability_css = document.createElement('link')
_readability_css.rel = 'stylesheet'
_readability_css.href = 'http://sean.whitton.me/geek/old-readability.css'
_readability_css.type = 'text/css'
_readability_css.media = 'all'
document.documentElement.appendChild(_readability_css)
var _readability_print_css = document.createElement('link')
_readability_print_css.rel = 'stylesheet'
_readability_print_css.href = 'http://sean.whitton.me/geek/old-readability-print.css'
_readability_print_css.media = 'print'
_readability_print_css.type = 'text/css'
document.getElementsByTagName('head')[0].appendChild(_readability_print_css)
});
define_key(content_buffer_normal_keymap, "C-x C-r", "readability_arc90");