How to make an overlay for an image inside HTML text in NSAttributedString using CSS

I’m trying to parse html text that contains a youtube video. To display this video I use a preview image.

Initial text:

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>\n<p><iframe loading=\"lazy\" title=\"Forza Horizon 5 : RTX 4090 24GB ( 8K Maximum Settings RTX ON / DLSS ON )\" width=\"500\" height=\"281\" src=\"https://www.youtube.com/embed/NcHyE_N1QDI?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" allowfullscreen></iframe></p>\n<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>

And then it looks like these:

<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. </p>\n<p><a href=\'https://www.youtube.com/watch?v=NcHyE_N1QDI\'><img src=\"https://img.youtube.com/vi/NcHyE_N1QDI/maxresdefault.jpg\" alt=\"\" width=\"393.0\"/></a></p>\n<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>

Then I use NSMutableAttributedString and assign new value to the UITextView’s attributedText property.

The question is how to overlay an image with a play button on top of the preview image so that the user understands that this is a video. I assume this can be done using CSS, but don’t know how to properly embed it in the code.

My code:

private func formatStringWithYTVideo(text: String, with width: Float) -> String {
        let iframeTexts = matches(for: ".*iframe.*", in: text)
        var newText = text
        
        if !iframeTexts.isEmpty {
            
            for iframeText in iframeTexts {
                let iframeId = matches(for: "((?<=(v|V)/)|(?<=be/)|(?<=(\\?|\\&)v=)|(?<=embed/))([\\w-]++)", in: iframeText);
                
                if !iframeId.isEmpty {
                    let htmlString = """
                    <p><a href="https://www.youtube.com/watch?v=\(iframeId[0])"><img src="https://img.youtube.com/vi/\(iframeId[0])/maxresdefault.jpg" alt="" width="\(width)"/></a></p>
                    """
                    
                    newText = newText.replacingOccurrences(of: iframeText, with: htmlString)
                }
            }
        }
        
        return newText
    }