HTML:
Codebehind:
MetaDescription.Attr
You should remove the id property from the , this means the control won't be accessible from the server side by id but instead of it it could be dynamically created and then added to the page:
HtmlMeta meta = new HtmlMeta();
meta.Name = "keywords";
meta.Content = ThisBlog.MetaDescription;
this.Header.Controls.Add(meta);