Cannot set text to System Font in an NSAttributedString with CSS styling

I've got the following extension to turn an API response in html format into a NSAttributedString, styled with CSS:

extension String {
public func convertHtmlToAttributedStringWithCSS(bodyFont: UIFont, h1Font: UIFont, csscolor: String, lineheight: Int, csstextalign: String) -> NSAttributedString? {
        let modifiedString =
        "<style>" +
            "body{" +
                "font-family: '\(bodyFont.fontName)'; " +
                "font-size:\(bodyFont.pointSize)px; " +
                "color: \(csscolor); " +
                "line-height: \(lineheight)px; " +
                "text-align: \(csstextalign); " +
            "} " +
            "h1{" +
                "font-family: '\(h1Font.fontName)'; " +
                "font-size:\(h1Font.pointSize)px; " +
            "}" +
        "</style>\(self)";
        guard let data = modifiedString.data(using: .utf8) else {
            return nil
        }
        do {
            return try NSAttributedString(data: data, options: [.documentType: NSAttributedString.DocumentType.html, .characterEncoding: String.Encoding.utf8.rawValue], documentAttributes: nil)
        }
        catch {
            print(error)
            return nil
        }
    }
}

I'm calling it like so:

self.CampaignDescriptionBody.attributedText = briefing!.convertHtmlToAttributedStringWithCSS(bodyFont: UIFont.systemFont(ofSize: 12), h1Font: UIFont.systemFont(ofSize: 15), csscolor: "grey", lineheight: 5, csstextalign: "left")

Fontsize, textalignment, etc works fine. My issue is, that the font on screen does not at all look like the System Font, which seems to be San Francisco now.

Here's a screenshot of 2 UILabels that sit next to each other on screen. "Campaign Description" is set to "System Bold 15" in the Interface Builder "Exklusivität" is a piece of the API response that is turned into an NSAttributedString, so it should render with the same font but obviously doesn't.

Does anyone know what I'm doing wrong here?