HTML to NSAttributedString and NSAttributedString to HTML

I solved this by applying 0.75 ratio on each fond size in your string. Say if you have multiple fonts in your attributed string, when you looping though all of them, simple apply the ratio and then you are all set. Here is my code in swift 3.0:

yourAttrStr.beginEditing()
yourAttrStr.enumerateAttribute(NSFontAttributeName, in: NSMakeRange(0, yourAttrStr.length), options: .init(rawValue: 0)) { 
        (value, range, stop) in
        if let font = value as? UIFont {
            let resizedFont = font.withSize(font.pointSize * 0.75)
            yourAttrStr.addAttribute(NSFontAttributeName, value: resizedFont, range: range)
        }
}
yourAttrStr.endEditing()//yourAttrStr will be the same size as html string

This is the piece of code that is running in my application

extension String {
    func htmlAttributedString() -> NSAttributedString? {
        guard let data = self.data(using: String.Encoding.utf16, allowLossyConversion: false) else { return nil }
        guard let html = try? NSMutableAttributedString(
            data: data,
            options: [NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType],
            documentAttributes: nil) else { return nil }
        html.beginEditing()
        html.enumerateAttribute(NSFontAttributeName, in: NSMakeRange(0, html.length), options: .init(rawValue: 0)) {
            (value, range, stop) in
            if let font = value as? UIFont {
                let resizedFont = font.withSize(font.pointSize * 0.75)
                html.addAttribute(NSFontAttributeName,
                                         value: resizedFont,
                                         range: range)
            }
        }
        html.endEditing()
        return html
    }
}

To use it:

let htmlStr: String = "<font size=\"6\">font a</font><font size=\"16\">Font b</font>"
let attriStr: NSAttributedString? = htmlStr.htmlAttributedString()