Enhance Trello-cards with markdown based description with this Greasemonkey script.

trello-card-enhancer.js

// ==UserScript==
// @name         Trello Card Enhancer
// @namespace    https://gist.github.com/phdd/f3e8cf831c1c0b6fe324
// @description  Enhance cards with markdown based description
// @copyright    2015+, Peter Heisig
// @version      1.3
// @updateURL    https://gist.githubusercontent.com/raw/f3e8cf831c1c0b6fe324/trello-card-enhancer.js
//
// @match        https://trello.com/*
// @match        https://*.trello.com/*
//
// @require      https://cdnjs.cloudflare.com/ajax/libs/marked/0.3.2/marked.min.js
// @require      https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js
// @require      https://api.trello.com/1/client.js?key=5fd1b9324345b17684be897ba58c5752
// @require      https://raw.github.com/sizzlemctwizzle/GM_config/master/gm_config.js
//
// @grant        GM_addStyle
// @grant        GM_getValue
// @grant        GM_setValue
// @grant        GM_registerMenuCommand
//
// ==/UserScript==

GM_registerMenuCommand('Trello Card Enhancer Settings', function() {
    GM_config.open();
});

GM_config.init({
    'id': 'TrelloCardEnhancer',
    'fields': {
        'ListWidth': {
            'label': 'Board list width',
            'type': 'int',
            'default': '270'
        },
        'LogoutAction': {
            'label': 'Disconnect Trello',
            'type': 'button',
            'size': 100,
            'click': Trello.deauthorize
        }
    },
    'events': {
        'save': function () { initialize(); GM_config.close(); },
        'reset': initialize
    }
});

function initializeCss() {
    var width = GM_config.get('ListWidth');

    GM_addStyle('.list-card .markeddown li { margin-left: 0px !important; }');
    GM_addStyle('.list-card .markeddown ul,' +
                '.list-card .markeddown ol { margin-left: 20px !important; }');

    GM_addStyle('.list { width: ' + width + 'px !important; max-width: ' + width + 'px !important; flex: 0 0 ' + width + 'px !important; }');
    GM_addStyle('.list-wrapper { width: ' + width + 'px !important; }');
    GM_addStyle('.list-card { max-width: ' + width + 'px !important; }');
    GM_addStyle('.list-card img { max-width: 100% !important; }');
}

function enhanceCard(response, idMap) {
    var card = idMap[idFromShortUrl(response.shortUrl)];
    var markdown = "### " + response.name + "\n" + response.desc;

    $(card).html(marked(markdown));
}

function enhanceCards(idMap) {
    $($('.icon-description')).parent().remove();

    $('.list-card-title').each(function(index, item) {
        $(item).addClass('markeddown');
    });

    var urls = [];
    for (var id in idMap) {
        urls.push("/cards/" + id);
    }

    Trello.get('batch', { gmRequest: true, urls: urls.join(',') }, function(responses) {
        $(responses).each(function(index, response) {
            enhanceCard(response[200], idMap);
        });
    });
}

function initializeCards() {
    var idMap = {};

    $('.list-card-title[href]').each(function(index, item) {
        var path = $(item).attr('href');
        var cardId = idFromShortPath(path);

        idMap[cardId] = item;
    });

    enhanceCards(idMap);
}

function initialize() {
    initializeCss();
    initializeCards();
}

function authorize() {
    Trello.authorize({
        name: 'Card Enhancer',
        type: 'popup',
        success: initialize
    });
}

function isBoardPage() {
    return window.location.href.match(/trello\.com\/b\//);
}

function enhance() {
    if (!Trello.authorized()) {
        authorize();
    } else {
        initialize();
    }
}

$(document).ready(function() {
    if (isBoardPage()) {
        enhance();
    }
});

function idFromShortPath(path) {
    return path.split('/')[2];
}

function idFromShortUrl(url) {
    return url.split('/')[4];
}

function updateFrom(response) {
    if (defined(response.shortUrl)) {
        var shortId = idFromShortUrl(response.shortUrl);

        if (typeof response.id !== "undefined") {
            var card = $(".list-card-title[href*='" + shortId + "']")[0];
            var idMap = {};
            idMap[shortId] = card;
            enhanceCards(idMap);
        }
    }
}

function defined(val) {
    return typeof val !== "undefined";
}

function monitor(request, args) {
    if ( request.responseURL.match(/\/1\/cards/) &&
        !request.responseURL.match(/gmRequest=true/)
    ) {
        var response = $.parseJSON(request.responseText);

        if (defined(response)) {
            updateFrom(response);
        }

    } else if (request.responseURL.match(/markAsViewed/)) {
        initializeCards();
    }
}

(function() {
    var proxied = window.XMLHttpRequest.prototype.send;
    window.XMLHttpRequest.prototype.send = function() {
        var sendArguments = arguments;
        //Here is where you can add any code to process the request.
        //If you want to pass the Ajax request object, pass the 'pointer' below
        var pointer = this;
        var intervalId = window.setInterval(function() {
            if (pointer.readyState != 4) {
                return;
            }

            if (pointer.status == 200) {
                monitor(pointer, sendArguments);
            }

            //Here is where you can add any code to process the response.
            //If you want to pass the Ajax request object, pass the 'pointer' below
            clearInterval(intervalId);

        }, 1); //I found a delay of 1 to be sufficient, modify it as you need.

        return proxied.apply(this, [].slice.call(arguments));
    };
})();