Insert CSS into loaded HTML in UIWebView / WKWebView

Solution 1:

You can do it like this:

- (void)webViewDidFinishLoad:(UIWebView *)webView {
    NSString *cssString = @"body { font-family: Helvetica; font-size: 50px }"; // 1
    NSString *javascriptString = @"var style = document.createElement('style'); style.innerHTML = '%@'; document.head.appendChild(style)"; // 2
    NSString *javascriptWithCSSString = [NSString stringWithFormat:javascriptString, cssString]; // 3
    [webView stringByEvaluatingJavaScriptFromString:javascriptWithCSSString]; // 4
}

What this code does:

// 1 : Define a string that contains all the CSS declarations

// 2 : Define a javascript string that creates a new <style> HTML DOM element and inserts the CSS declarations into it. Actually the inserting is done in the next step, right now there is only the %@ placeholder. I did this to prevent the line from becoming too long, but step 2 and 3 could be done together.

// 3 : Combine the 2 strings

// 4 : Execute the javascript in the UIWebView

For this to work, your HTML has to have a <head></head> element.

EDIT:

You can also load the css string from a local css file (named "styles.css" in this case). Just replace step //1 with the following:

NSString *path = [[NSBundle mainBundle] pathForResource:@"styles" ofType:@"css"];
NSString *cssString = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];

As another option you can just inject a <link> element to the <head> that loads the CSS file:

- (void)webViewDidFinishLoad:(UIWebView *)webView {
    NSString *path = [[NSBundle mainBundle] pathForResource:@"styles" ofType:@"css"];
    NSString *javascriptString = @"var link = document.createElement('link'); link.href = '%@'; link.rel = 'stylesheet'; document.head.appendChild(link)";
    NSString *javascriptWithPathString = [NSString stringWithFormat:javascriptString, path];
    [webView stringByEvaluatingJavaScriptFromString:javascriptWithPathString];
}

This solution works best for large CSS files. Unfortunately it does not work with remote HTML files. You can only use this when you want to insert CSS into HTML that you have downloaded to your app.

UPDATE: WKWebView / Swift 3.x

When you are working with a WKWebView injecting a <link> element does not work because of WKWebView's security settings.

You can still inject the css as a string. Either create the CSS string in your code //1 or put it in a local file //2. Just be aware that with WKWebView you have to do the injection in WKNavigationDelegate's webView(_:didFinish:) method:

func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
    insertCSSString(into: webView) // 1
    // OR
    insertContentsOfCSSFile(into: webView) // 2
}

func insertCSSString(into webView: WKWebView) {
    let cssString = "body { font-size: 50px; color: #f00 }"
    let jsString = "var style = document.createElement('style'); style.innerHTML = '\(cssString)'; document.head.appendChild(style);"
    webView.evaluateJavaScript(jsString, completionHandler: nil)
}

func insertContentsOfCSSFile(into webView: WKWebView) {
    guard let path = Bundle.main.path(forResource: "styles", ofType: "css") else { return }
    let cssString = try! String(contentsOfFile: path).trimmingCharacters(in: .whitespacesAndNewlines)
    let jsString = "var style = document.createElement('style'); style.innerHTML = '\(cssString)'; document.head.appendChild(style);"
    webView.evaluateJavaScript(jsString, completionHandler: nil)
}

Solution 2:

Since UIWebView is deprecated in iOS 12, I'll only answer for WKWebView. I've implemented CSS loading like it was described in the accepted answer. The problem was that sometimes the transition from HTML with no CSS applied to HTML with CSS was visible.
I think a better approach is to use the WKUserScript to inject the CSS like this:

lazy var webView: WKWebView = {
    guard let path = Bundle.main.path(forResource: "style", ofType: "css") else {
        return WKWebView()
    }

    let cssString = try! String(contentsOfFile: path).components(separatedBy: .newlines).joined()
    let source = """
      var style = document.createElement('style');
      style.innerHTML = '\(cssString)';
      document.head.appendChild(style);
    """

    let userScript = WKUserScript(source: source,
                                  injectionTime: .atDocumentEnd,
                                  forMainFrameOnly: true)

    let userContentController = WKUserContentController()
    userContentController.addUserScript(userScript)

    let configuration = WKWebViewConfiguration()
    configuration.userContentController = userContentController

    let webView = WKWebView(frame: .zero,
                            configuration: configuration)
    return webView
}()

You can read more about this approach in this blog post.

Solution 3:

Instead of applying css with style tag it's better to apply it with the link tag:

func insertContentsOfCSSFile2(into webView: WKWebView) {
    guard let path = Bundle.main.path(forResource: "resource", ofType: "css") else { return }                
    let csFile = "var head = document.getElementsByTagName('head')[0];var link = document.createElement('link'); link.rel = 'stylesheet';link.type = 'text/css';link.href = '\(path)';link.media = 'all';head.appendChild(link);"

    webView.evaluateJavaScript(csFile) {(result, error) in
        if let error = error {
            print(error)
        }
    }
}

I have tested it. it is working fine.

Solution 4:

You need to add this header before apple style to HTML

let fontName = UIFont.systemFont(ofSize: 17.0).fontName
    let htmlContent = """
    <header>
    <meta name='viewport' content='width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no'>
    </header>
    <style type='text/css'>
    img{max-height: 100%; min-height: 100%; height:auto; max-width: 100%; width:auto;margin-bottom:5px;}
    p{text-align:left|right|center; line-height: 180%; font-family: '\(fontName)'; font-size: 17px;}
    iframe{width:100%; height:250px;}
    </style> \(html)
    """
    webView.loadHTMLString(htmlContent, baseURL: Bundle.main.bundleURL)

Insert CSS into loaded HTML in UIWebView / WKWebView

Solution 1:

Solution 2:

Solution 3:

Solution 4:

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