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");