在Body中追加 代码：

    Private Sub cmdAddPic_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles cmdAddPic.Click
        Dim el As HtmlElement = web.Document.CreateElement("div")
        el.InnerHtml = "<a href=''>test</a>"
        el.Id = "id1"
        web.Document.Body.AppendChild(el)
        el = Nothing
    End Sub

在指定的标签里追加代码

    Private Sub cmdAddPic_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles cmdAddPic.Click
        el = web.Document.CreateElement("div")
        el.InnerHtml = "test2"
        el.Id = "id2"
        web.Document.GetElementById("id1").AppendChild(el)
        el = Nothing
    End Sub

其实就是先通过GetElementById找到标签，然后再追加到进去