Custom Font Not Working in WKWebView Swift

Reading the answers in the linked thread in DonMag's comment:

• Using @font-face is mandatory

• You need multiple @font-face declarations to use multiple font files as a single font family

• You need to provide baseURL to make relative urls like url(OpenSans-Regular.ttf) work

So, try this:

    let htmlString = """
    <style>
    @font-face
    {
        font-family: 'Open Sans';
        font-weight: normal;
        src: url(OpenSans-Regular.ttf);
    }
    @font-face
    {
        font-family: 'Open Sans';
        font-weight: bold;
        src: url(OpenSans-Bold.ttf);
    }
    @font-face
    {
        font-family: 'Open Sans';
        font-weight: 900;
        src: url(OpenSans-ExtraBold.ttf);
    }
    @font-face
    {
        font-family: 'Open Sans';
        font-weight: 200;
        src: url(OpenSans-Light.ttf);
    }
    @font-face
    {
        font-family: 'Open Sans';
        font-weight: 500;
        src: url(OpenSans-Semibold.ttf);
    }
    </style>
    <span style="font-family: 'Open Sans'; font-weight: bold; font-size: 30; color: red">(Utils.aboutUsText)</span>
    """
    webView.loadHTMLString(htmlString, baseURL: Bundle.main.bundleURL) //<- 

---

Or you can use a separate css file if you prefer:

    let htmlString = """
    <link rel="stylesheet" type="text/css" href="open-sans.css">
    <span style="font-family: 'Open Sans'; font-weight: bold; font-size: 30; color: red">(Utils.aboutUsText)</span>
    """
    webView.loadHTMLString(htmlString, baseURL: Bundle.main.bundleURL)

open-sans.css:

@font-face
{
    font-family: 'Open Sans';
    font-weight: normal;
    src: url(OpenSans-Regular.ttf);
}
@font-face
{
    font-family: 'Open Sans';
    font-weight: bold;
    src: url(OpenSans-Bold.ttf);
}
@font-face
{
    font-family: 'Open Sans';
    font-weight: 900;
    src: url(OpenSans-ExtraBold.ttf);
}
@font-face
{
    font-family: 'Open Sans';
    font-weight: 200;
    src: url(OpenSans-Light.ttf);
}
@font-face
{
    font-family: 'Open Sans';
    font-weight: 500;
    src: url(OpenSans-Semibold.ttf);
}