Html inside label using Html helper

Rather then writing an extension method you could use the following razor code:

@{ MvcHtmlString label = Html.LabelFor(m => m.ModelProperty, "<span class='cssClass'>Label HTML</span>", new { @class = "clabel"}); }
@Html.Raw(HttpUtility.HtmlDecode(label.ToString()))

It's not as clean but if you need something quick it works.

Looks like a good scenario for a custom helper:

public static class LabelExtensions
{
    public static MvcHtmlString LabelFor<TModel, TProperty>(
        this HtmlHelper<TModel> htmlHelper, 
        Expression<Func<TModel, TProperty>> ex, 
        Func<object, HelperResult> template
    )
    {
        var htmlFieldName = ExpressionHelper.GetExpressionText(ex);
        var for = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(htmlFieldName);
        var label = new TagBuilder("label");
        label.Attributes["for"] = TagBuilder.CreateSanitizedId(for);
        label.InnerHtml = template(null).ToHtmlString();
        return MvcHtmlString.Create(label.ToString());
    }
}

and then:

@Html.LabelFor(
    x => x.Name, 
    @<span>Hello World</span>
)

---

UPDATE:

To achieve what you asked in the comments section you may try the following:

public static class HtmlHelperExtensions
{
    public static MvcHtmlString LabelFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> ex, Func<object, HelperResult> template)
    {
        var htmlFieldName = ExpressionHelper.GetExpressionText(ex);
        var propertyName = htmlFieldName.Split('.').Last();
        var label = new TagBuilder("label");
        label.Attributes["for"] = TagBuilder.CreateSanitizedId(htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(htmlFieldName));
        label.InnerHtml = string.Format(
            "{0} {1}", 
            propertyName,
            template(null).ToHtmlString()
        );
        return MvcHtmlString.Create(label.ToString());
    }
}

and then:

@Html.LabelFor(
    x => x.Name, 
    @<em>mandatory</em>
)