Code listings for blog post about rendering EPiServer CMS properties with dynamic HTML attributes. Read about it here: https://stefanolsen.com/posts/adding-dynamic-html-to-property-templates-in-episerver/

ContentReference.cshtml

@model EPiServer.Core.ContentReference
@Html.ContentLink(Model, null, Html.LinkAttributes(Model))

HtmlHelperExtensions.cs

public static class HtmlHelperExtensions
{
    public static RouteValueDictionary LinkAttributes(this HtmlHelper helper, ContentReference contentLink)
    {
        var resolver = ServiceLocator.Current.GetInstance<LinkAttributesResolver>();
        var attributes = resolver.ToLinkAttributes(contentLink);

        return attributes;
    }

    public static RouteValueDictionary LinkAttributes(this HtmlHelper helper, Url url, string action = null)
    {
        var resolver = ServiceLocator.Current.GetInstance<LinkAttributesResolver>();
        var attributes = resolver.ToLinkAttributes(url, action);

        return attributes;
    }

    public static MvcHtmlString InsertLinkAttributes(this HtmlHelper helper, ContentReference contentLink)
    {
        var resolver = ServiceLocator.Current.GetInstance<LinkAttributesResolver>();
        var attributes = resolver.ToLinkAttributes(contentLink);

        return MvcHtmlString.Create(string.Join(" ", attributes.Select(a => $"{a.Key}=\"{a.Value}\"")));
    }

    public static MvcHtmlString InsertLinkAttributes(this HtmlHelper helper, Url url, string action = null)
    {
        var resolver = ServiceLocator.Current.GetInstance<LinkAttributesResolver>();
        var attributes = resolver.ToLinkAttributes(url, action);

        return MvcHtmlString.Create(string.Join(" ", attributes.Select(a => $"{a.Key}=\"{a.Value}\"")));
    }

    public static MvcHtmlString InsertLinkAttributes(this HtmlHelper helper, MvcHtmlString xhtmlString)
    {
        var resolver = ServiceLocator.Current.GetInstance<LinkAttributesResolver>();

        return MvcHtmlString.Create(resolver.InsertLinkAttributes(xhtmlString));
    }
}

LinkAttributesResolver.cs

public class LinkAttributesResolver
{
    private const string DataCategory = "data-category";
    private const string DataLabel = "data-label";
    private const string DataAction = "data-action";
    private const string Download = "download";
    private const string Target = "target";

    private const string AttributesCategoryExternal = "External";
    private const string AttributesCategoryDownload = "Downloads";
    private const string AttributeDownload = "download";
    private const string AttributesTargetBlank = "_blank";

    private static readonly Regex ExternalLinkRegex =
        new Regex(@"<a(?:.*?)href=""(?<linkUrl>(?:(https?|mailto):)(?:.*?))""(?:.*?)>", RegexOptions.Compiled);
    private static readonly Regex MediaLinkRegex =
        new Regex(@"<a(?:.*?)href=""(?<linkUrl>(?:(\/globalassets\/|\/siteassets\/|\/contentassets\/))(?:.*?))""(?:.*?)(?:target=""(?<target>.*)"")?>", RegexOptions.Compiled);

    private readonly IContentLoader _contentLoader;
    private readonly ISiteDefinitionResolver _siteDefinitionResolver;
    private readonly UrlResolver _urlResolver;

    public LinkAttributesResolver(
        IContentLoader contentLoader,
        ISiteDefinitionResolver siteDefinitionResolver,
        UrlResolver urlResolver)
    {
        _contentLoader = contentLoader;
        _siteDefinitionResolver = siteDefinitionResolver;
        _urlResolver = urlResolver;
    }

    public RouteValueDictionary ToLinkAttributes(ContentReference contentLink)
    {
        var result = new RouteValueDictionary();

        if (ContentReference.IsNullOrEmpty(contentLink))
        {
            return result;
        }

        IContent content;
        if (!_contentLoader.TryGet(contentLink, out content))
        {
            return result;
        }

        IContent shortcutContent;
        if (TryGetShortcut(content, out shortcutContent))
        {
            content = shortcutContent;
        }

        string action = string.Empty;
        string category = string.Empty;
        string label = string.Empty;
        string download = string.Empty;

        var mediaData = content as MediaData;
        if (mediaData != null)
        {
            category = AttributesCategoryDownload;
            label = mediaData.Name;
            action = GetFileExtension(mediaData);

            if (mediaData is ImageFile)
            {
                download = mediaData.Name;
            }
        }
        else
        {
            var siteDefinition = _siteDefinitionResolver.GetByContent(content.ContentLink, false, false);
            if (siteDefinition == SiteDefinition.Empty || siteDefinition != SiteDefinition.Current)
            {
                category = AttributesCategoryExternal;
                label = GetFriendlyUrl(contentLink);
                action = AttributesCategoryExternal;
            }
        }

        result.Add(DataCategory, category);
        result.Add(DataLabel, label);
        result.Add(DataAction, action);

        if (!string.IsNullOrEmpty(download))
        {
            result.Add(Download, download);
        }

        return result;
    }

    public RouteValueDictionary ToLinkAttributes(Url url, string action)
    {
        var result = new RouteValueDictionary();

        if (url == null || url.IsEmpty())
        {
            return result;
        }

        if (!url.IsAbsoluteUri)
        {
            IContent content = GetContent(url);

            return content == null ? result : ToLinkAttributes(content.ContentLink);
        }

        string category = string.Empty;
        string label = string.Empty;
        string download = string.Empty;

        if (url.ToString().StartsWith("mailto:"))
        {
            category = "Contact";
            label = url.ToString().Substring(7);
            action = "mailto";
        }
        else
        {
            category = AttributesCategoryExternal;
            label = GetFriendlyUrl(url);
            action = !string.IsNullOrEmpty(action) ? action : AttributesCategoryExternal;
        }

        result.Add(DataCategory, category);
        result.Add(DataLabel, label);
        result.Add(DataAction, action);

        if (!string.IsNullOrEmpty(download))
        {
            result.Add(Download, download);
        }

        return result;
    }

    /// <summary>
    /// Inserts computed attributes for all links in an HTML string.
    /// </summary>
    public string InsertLinkAttributes(MvcHtmlString xhtmlString)
    {
        string htmlString = xhtmlString.ToHtmlString();

        var externalLinkMatchEvaluator = new MatchEvaluator(ExternalLinkMatchEvaluator);
        string replaced = ExternalLinkRegex.Replace(htmlString, externalLinkMatchEvaluator);

        var mediaLinkMatchEvaluator = new MatchEvaluator(MediaLinkMatchEvaluator);
        replaced = MediaLinkRegex.Replace(replaced, mediaLinkMatchEvaluator);

        return replaced;
    }

    /// <summary>
    /// Tries to lookup an external looking URL, and add the computed attributes.
    /// </summary>
    private string ExternalLinkMatchEvaluator(Match match)
    {
        string linkTag = match.Value;
        string linkUrl = match.Groups["linkUrl"].Value;
        bool hasTarget = match.Groups["target"].Success;

        RouteValueDictionary attributes = ToLinkAttributes(new Url(linkUrl), AttributesCategoryExternal);
        if (hasTarget)
        {
            attributes.Remove(Target);
        }

        var attributeString = MvcHtmlString.Create(string.Join(" ", attributes.Select(a => $"{a.Key}=\"{a.Value}\"")));

        linkTag = linkTag.Replace(">", $" {attributeString}>");

        return linkTag;
    }

    /// <summary>
    /// Tries to lookup a media looking URL, and add the computed attributes.
    /// </summary>
    private string MediaLinkMatchEvaluator(Match match)
    {
        string linkTag = match.Value;
        string linkUrl = match.Groups["linkUrl"].Value;
        bool hasTarget = match.Groups["target"].Success;

        RouteValueDictionary attributes =
            ToLinkAttributes(new Url(linkUrl), AttributesCategoryDownload);
        if (hasTarget)
        {
            attributes.Remove(Target);
        }

        var attributeString =
            MvcHtmlString.Create(string.Join(" ", attributes.Select(a => $"{a.Key}=\"{a.Value}\"")));

        linkTag = linkTag.Replace(">", $" {attributeString}>");

        return linkTag;
    }

    private bool TryGetShortcut(IContent content, out IContent shortcut)
    {
        shortcut = null;

        var page = content as PageData;
        if (page == null || page.LinkType != PageShortcutType.Shortcut)
        {
            return false;
        }

        var propertyContentReference = page.Property["PageShortcutLink"] as PropertyContentReference;
        if (propertyContentReference != null &&
            !ContentReference.IsNullOrEmpty(propertyContentReference.ContentLink))
        {
            _contentLoader.TryGet(propertyContentReference.ContentLink, out shortcut);
            return true;
        }

        return false;
    }

    private string GetFriendlyUrl(Url internalUrl)
    {
        if (internalUrl == null || internalUrl.IsEmpty())
        {
            return string.Empty;
        }

        var url = new UrlBuilder(internalUrl);
        return _urlResolver.GetUrl(url, new VirtualPathArguments { ContextMode = ContextMode.Default }) ?? string.Empty;
    }

    private string GetFriendlyUrl(ContentReference contentReference)
    {
        if (ContentReference.IsNullOrEmpty(contentReference))
        {
            return string.Empty;
        }

        return _urlResolver.GetUrl(contentReference);
    }

    private IContent GetContent(Url internalUrl)
    {
        if (internalUrl == null)
        {
            return default(IContent);
        }

        var url = new UrlBuilder(internalUrl);
        return _urlResolver.Route(url);
    }

    private string GetFileExtension(MediaData mediaData)
    {
        if (mediaData == null)
        {
            return string.Empty;
        }

        string url = GetFriendlyUrl(mediaData.ContentLink);
        return url.Split('.').LastOrDefault();
    }
}

Url.cshtml

@model EPiServer.Url
@Html.ContentLink(Model, Html.LinkAttributes(Model))

XhtmlString.cshtml

@model EPiServer.Core.XhtmlString
@{
    MvcHtmlString xhtmlString = Html.XhtmlString(Model);

    xhtmlString = Html.InsertLinkAttributes(xhtmlString);
}
@xhtmlString