你可以通过一点编码来完成它...你需要遍历属性，并改变属性化字符串的每一段的字体大小（如正常、斜体、粗体等）。

extension String {
    func htmlAttributedString() -> NSMutableAttributedString {

            guard let data = self.data(using: String.Encoding.utf8, allowLossyConversion: false)
                else { return NSMutableAttributedString() }

            guard let formattedString = try? NSMutableAttributedString(data: data,
                                                            options: [.documentType: NSAttributedString.DocumentType.html,
                                                                      .characterEncoding: String.Encoding.utf8.rawValue],
                                                            documentAttributes: nil )

                else { return NSMutableAttributedString() }

            return formattedString
    }

}

extension NSMutableAttributedString {

    func with(font: UIFont) -> NSMutableAttributedString {
        self.enumerateAttribute(NSAttributedStringKey.font, in: NSMakeRange(0, self.length), options: .longestEffectiveRangeNotRequired, using: { (value, range, stop) in
            let originalFont = value as! UIFont
            if let newFont = applyTraitsFromFont(originalFont, to: font) {
                self.addAttribute(NSAttributedStringKey.font, value: newFont, range: range)
            }
        })
        return self
    }

    func applyTraitsFromFont(_ f1: UIFont, to f2: UIFont) -> UIFont? {
        let originalTrait = f1.fontDescriptor.symbolicTraits

        if originalTrait.contains(.traitBold) {
            var traits = f2.fontDescriptor.symbolicTraits
            traits.insert(.traitBold)
            if let fd = f2.fontDescriptor.withSymbolicTraits(traits) {
                return UIFont.init(descriptor: fd, size: 0)
            }
        }
        return f2
    }
}

你可以这样使用它

let formattedString = "text  <b>sometext</b>"
                        .htmlAttributedString()
                        .with(font:UIFont(name: "Times New Roman", size: 37)!)

下面是@Maria的NSMutableAttributedString扩展名在Swift 5.1中的重写

extension NSMutableAttributedString {

    func with(font: UIFont) -> NSMutableAttributedString {
        enumerateAttribute(NSAttributedString.Key.font, in: NSMakeRange(0, length), options: .longestEffectiveRangeNotRequired, using: { (value, range, stop) in
            if let originalFont = value as? UIFont, let newFont = applyTraitsFromFont(originalFont, to: font) {
                addAttribute(NSAttributedString.Key.font, value: newFont, range: range)
            }
        })

        return self
    }

    func applyTraitsFromFont(_ originalFont: UIFont, to newFont: UIFont) -> UIFont? {
        let originalTrait = originalFont.fontDescriptor.symbolicTraits

        if originalTrait.contains(.traitBold) {
            var traits = newFont.fontDescriptor.symbolicTraits
            traits.insert(.traitBold)

            if let fontDescriptor = newFont.fontDescriptor.withSymbolicTraits(traits) {
                return UIFont.init(descriptor: fontDescriptor, size: 0)
            }
        }

        return newFont
    }
}

这是我的版本，支持文本样式（正文、标题等）、文本对齐，可直接在UILabel中使用：
UILabel扩展：

import UIKit

extension UILabel {

    // See: https://stackoverflow.com/questions/34263767/can-i-set-default-font-when-translating-html-into-nsattributedstring
    func set(html: String, textAligment: NSTextAlignment = NSTextAlignment.natural) {
        let textStyle = self.font.fontDescriptor.object(forKey: UIFontDescriptor.AttributeName.textStyle) as! UIFont.TextStyle

        do {
            self.attributedText = try html.htmlAttributedString(textAligment: textAligment).with(textStyle: textStyle)
        } catch let e {
            print("Couldn\'t parse \'\(html)\': \(e.localizedDescription)")
        }
    }
}

String扩展：

import Foundation

extension String {

    enum StringToHTMLError: Error {
        case encoding
    }

    func htmlAttributedString(textAligment: NSTextAlignment = NSTextAlignment.natural) throws -> NSMutableAttributedString {
        guard let data = self.data(using: String.Encoding.utf8, allowLossyConversion: false) else {
            throw StringToHTMLError.encoding
        }

        let text = try NSMutableAttributedString(data: data,
                                             options: [.documentType: NSAttributedString.DocumentType.html,
                                                       .characterEncoding: String.Encoding.utf8.rawValue],
                                             documentAttributes: nil )

        // See: https://stackoverflow.com/a/33443521/5189200
        let style = NSMutableParagraphStyle()
        style.alignment = textAligment
        text.addAttributes([.paragraphStyle: style],
                           range: NSMakeRange(0, text.length))

        return text
    }
}

和NSMutableAttributedString扩展名：

import Foundation

extension NSMutableAttributedString {

    func with(textStyle: UIFont.TextStyle) -> NSMutableAttributedString {
        self.enumerateAttribute(NSAttributedString.Key.font, in: NSMakeRange(0, self.length), options: .longestEffectiveRangeNotRequired, using: { (value, range, stop) in
            let originalFont = value as! UIFont
            if let newFont = applyTraitsFromFont(originalFont, to: textStyle) {
                self.addAttribute(NSAttributedString.Key.font, value: newFont, range: range)
            }
        })
        return self
    }

    private func applyTraitsFromFont(_ f1: UIFont, to textStyle: UIFont.TextStyle) -> UIFont? {
        let originalTrait = f1.fontDescriptor.symbolicTraits
        let fontDescriptor = UIFontDescriptor.preferredFontDescriptor(withTextStyle: textStyle)

        if originalTrait.contains(.traitBold) {
            var traits = fontDescriptor.symbolicTraits
            traits.insert(.traitBold)
            if let fd = fontDescriptor.withSymbolicTraits(traits) {
                return UIFont.init(descriptor: fd, size: 0)
            }
        }

        return UIFont(descriptor: fontDescriptor, size: 0.0)
    }

    func with(font: UIFont) -> NSMutableAttributedString {
        enumerateAttribute(NSAttributedString.Key.font, in: NSMakeRange(0, length), options: .longestEffectiveRangeNotRequired, using: { (value, range, stop) in
            if let originalFont = value as? UIFont, let newFont = applyTraitsFromFont(originalFont, to: font) {
                addAttribute(NSAttributedString.Key.font, value: newFont, range: range)
            }
        })

        return self
    }

    private func applyTraitsFromFont(_ originalFont: UIFont, to newFont: UIFont) -> UIFont? {
        let originalTrait = originalFont.fontDescriptor.symbolicTraits

        if originalTrait.contains(.traitBold) {
            var traits = newFont.fontDescriptor.symbolicTraits
            traits.insert(.traitBold)

            if let fontDescriptor = newFont.fontDescriptor.withSymbolicTraits(traits) {
                return UIFont.init(descriptor: fontDescriptor, size: 0)
            }
        }

        return newFont
    }
}

用途：

<yourUILabel>.set(html: "Hello, <b>world</b>!",
                  textAligment: NSTextAlignment.center)

我希望这能帮上忙，
哈维

目标语言-C

在我的情况下，我只需要使默认字体变大。所以有解决方案。

- (NSAttributedString *)attributedStringFromHtml:(NSString *)htmlString largerFontBy:(CGFloat)pointSize fontColor:(UIColor *)fontColor
{
    NSData *data = [htmlString dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES];
    NSDictionary *options = @{
        NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType,
        NSCharacterEncodingDocumentAttribute: @(NSUTF8StringEncoding)
    };
    NSMutableAttributedString *mutableAttributedString = [[NSMutableAttributedString alloc] initWithData:data
                                                                                                 options:options
                                                                                      documentAttributes:nil
                                                                                                   error:nil];
    [mutableAttributedString beginEditing];
    [mutableAttributedString enumerateAttribute:NSFontAttributeName inRange:NSMakeRange(0, mutableAttributedString.length) options:0 usingBlock:^(id value, NSRange range, BOOL* stop)
     {
        if (value)
        {
            UIFont *oldFont = (UIFont*)value;
            UIFontDescriptor* fontDescriptor = [oldFont.fontDescriptor fontDescriptorWithSymbolicTraits:oldFont.fontDescriptor.symbolicTraits];
            UIFont *newFont = [UIFont fontWithDescriptor:fontDescriptor size:oldFont.pointSize + pointSize];
            [mutableAttributedString removeAttribute:NSFontAttributeName range:range];
            [mutableAttributedString addAttribute:NSFontAttributeName value:newFont range:range];
            [mutableAttributedString addAttribute:NSForegroundColorAttributeName value:fontColor range:range];
        }
    }];
    [mutableAttributedString endEditing];
    return mutableAttributedString;
}

如果你需要改变字体，这将是困难的，因为你需要新的字体，以支持所有traitsCollection的默认字体。你可以尝试玩行UIFontDescriptor* fontDescriptor = [oldFont.fontDescriptor fontDescriptorWithSymbolicTraits:oldFont.fontDescriptor.symbolicTraits];。使一个新的UIFontDescriptor的新字体，并创建字体与此描述符和特点的旧字体。在我的情况下，新字体是零，因为我没有所有必要的特点，为新字体。如粗体，斜体和等宽变体。