string str = a.ToString();
str = Regex.Replace(str, @"</?span[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"&#[^>]*;", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?marquee[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?object[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?param[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?embed[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?table[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @" ","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?tr[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?th[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?p[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?a[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?img[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?tbody[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?li[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?span[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?div[^>]*>","",RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?th[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?td[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"</?script[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"(javascript|jscript|vbscript|vbs):", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"on(mouse|exit|error|click|key)", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"<\\?xml[^>]*>", "", RegexOptions.IgnoreCase);
str = Regex.Replace(str, @"<\/?[a-z]+:[^>]*>", "", RegexOptions.Ignore
超文本标记语言（HTML）5 第一次向 HTML 中引入新的元素。新的结构元素包括 aside、figure 和 section。新的内联元素包括 time、meter 和 progress。新的内嵌元素有 video 和 audio。新的交互元素有 details、datagrid 和 command。
超文本标记语言（HTML）的开发到 ......
