How to use Javascript to check and load CSS if not loaded?

Just check to see if a <link> element exists with the href attribute set to your CSS file's URL:

if (!$("link[href='/path/to.css']").length)
    $('<link href="/path/to.css" rel="stylesheet">').appendTo("head");

The plain ol' JS method is simple too, using the document.styleSheets collection:

function loadCSSIfNotAlreadyLoadedForSomeReason () {
    var ss = document.styleSheets;
    for (var i = 0, max = ss.length; i < max; i++) {
        if (ss[i].href == "/path/to.css")
            return;
    }
    var link = document.createElement("link");
    link.rel = "stylesheet";
    link.href = "/path/to.css";

    document.getElementsByTagName("head")[0].appendChild(link);
}
loadCSSIfNotAlreadyLoadedForSomeReason();

I just had to write something like that and I wanted to share it. This one is prepared for multiple cases.

• If there is no request for the css file (css file isn't linked ...)
• If there is a request for the css file, but if it failed (css file is no longer available ...)

---

var styles = document.styleSheets;
for (var i = 0; i < styles.length; i++) {
    // checking if there is a request for template.css
    if (styles[i].href.match("template")) {
        console.log("(Iteration: " + i + ") Request for template.css is found.");
        // checking if the request is not successful
        // when it is successful .cssRules property is set to null
        if (styles[i].cssRules != null && styles[i].cssRules.length == 0) {
            console.log("(Iteration: " + i + ") Request for template.css failed.");
            // fallback, make your modification
            // since the request failed, we don't need to iterate through other stylesheets
            break;
        } else {
            console.log("(Iteration: " + i + ") Request for template.css is successful.");
            // template.css is loaded successfully, we don't need to iterate through other stylesheets
            break;
        }
    }
    // if there isn't a request, we fallback
    // but we need to fallback when the iteration is done
    // because we don't want to apply the fallback each iteration
    // it's not like our css file is the first css to be loaded
    else if (i == styles.length-1) {
        console.log("(Iteration: " + i + ") There is no request for template.css.");
        // fallback, make your modification
    }
}

TL;DR version

var styles = document.styleSheets;
for (var i = 0; i < styles.length; i++) {
    if (styles[i].href.match("css-file-name-here")) {
        if (styles[i].cssRules != null && styles[i].cssRules.length == 0) {
            // request for css file failed, make modification
            break;
        }
    } else if (i == styles.length-1) {
        // there is no request for the css file, make modification
    }
}

---

Update: Since my answer got a few upvotes and this led me to revise the code, I decided to update it.

// document.styleSheets holds the style sheets from LINK and STYLE elements
for (var i = 0; i < document.styleSheets.length; i++) {

    // Checking if there is a request for the css file
    // We iterate the style sheets with href attribute that are created from LINK elements
    // STYLE elements don't have href attribute, so we ignore them
    // We also check if the href contains the css file name
    if (document.styleSheets[i].href && document.styleSheets[i].href.match("/template.css")) {

        console.log("There is a request for the css file.");

        // Checking if the request is unsuccessful
        // There is a request for the css file, but is it loaded?
        // If it is, the length of styleSheets.cssRules should be greater than 0
        // styleSheets.cssRules contains all of the rules in the css file
        // E.g. b { color: red; } that's a rule
        if (document.styleSheets[i].cssRules.length == 0) {

            // There is no rule in styleSheets.cssRules, this suggests two things
            // Either the browser couldn't load the css file, that the request failed
            // or the css file is empty. Browser might have loaded the css file,
            // but if it's empty, .cssRules will be empty. I couldn't find a way to
            // detect if the request for the css file failed or if the css file is empty

            console.log("Request for the css file failed.");

            // There is a request for the css file, but it failed. Fallback
            // We don't need to check other sheets, so we break;
            break;
        } else {
            // If styleSheets.cssRules.length is not 0 (>0), this means 
            // rules from css file is loaded and the request is successful
            console.log("Request for the css file is successful.");
            break;
        }
    }
    // If there isn't a request for the css file, we fallback
    // But only when the iteration is done
    // Because we don't want to apply the fallback at each iteration
    else if (i == document.styleSheets.length - 1) {
        // Fallback
        console.log("There is no request for the css file.");
    }
}

TL;DR

for (var i = 0; i < document.styleSheets.length; i++) {
    if (document.styleSheets[i].href && document.styleSheets[i].href.match("/template.css")) {
        if (document.styleSheets[i].cssRules.length == 0) {
            // Fallback. There is a request for the css file, but it failed.
            break;
        }
    } else if (i == document.styleSheets.length - 1) {
        // Fallback. There is no request for the css file.
    }
}