Code
private void button1_Click(object sender, EventArgs e)
{
if (webBrowser1.Document.GetElementsByTagName("html")[0].OuterHtml.IndexOf(textBox1.Text) > 0)
MessageBox.Show(textBox1.Text + "找到了,在"+":"+webBrowser1.Document.GetElementsByTagName("html")[0].OuterHtml.IndexOf(textBox1.Text).ToString()+"的位置");
else
MessageBox.Show(textBox1.Text + "没有找到");
}