using System;

using System.Collections.Generic;

public class Program

{   

    public void Main()

    {

        string s = @"<p>This is a text</p><span class=""cls"" style=""background-color: yellow"">This is another text</span>";

        var list = s.Split(new[] {"<"}, StringSplitOptions.RemoveEmptyEntries);

        foreach (var item in list)

            Console.Write(ClearAttributes('<' + item));

        Console.ReadLine();

    }

    private static string ClearAttributes(string source)

    {

        int startindex = source.IndexOf('<');

        int endindex = source.IndexOf('>');

        string tag = source.Substring((startindex + 1), (endindex - startindex - 1));

        int spaceindex = tag.IndexOf(' ');

        if (spaceindex > 0)

            tag = tag.Substring(0, spaceindex);

        return String.Concat('<', tag, source.Substring(endindex));

    }

}