How to watermark and display user generated html in .NET - html

Question

I need to come up with a way to watermark user generated HTML and display it to the user.
I played around with modifying the HTML by setting a background image, but the solution didn't work if the any html element contained a background color or image.
So I thought it would be easier to display the HTML in a web browser control and watermark the control. Boy was I wrong.
I tried creating a user control and derive from WebBrowser and overwrite the paint method. That didn't work.
Next idea was to just draw on top of a webbrowser control.
here's what I've come up with. In this example I slapped a label, webbrowser control and two picture boxes on a form. I create the image and set it to both picture boxes. I put one picture box over the label and the other over the watermark.
The picturebox over the label displayed as I expected, but the picturebox over the webbrowser control did not.
I'm out of ideas. Any suggestions?
public partial class Form1 : Form
{
string WatermarkText = "Confidential";
Font WatermarkFont = new Font("Microsoft Sans Serif", 24, FontStyle.Bold);
int WatermarkAngle = 45;
Color WatermarkColor = Color.PaleTurquoise;
public Form1()
{
InitializeComponent();
AllowTransparency = true;
label1.AutoSize = false;
label1.Text = "Test Text";
label1.TextAlign = System.Drawing.ContentAlignment.MiddleCenter;
webBrowser1.Navigate("http://www.google.com");
var img = creategraphics(WatermarkText);
label1.Size = pictureBox2.Size = pictureBox1.Size = img.Size;
label1.Controls.Add(pictureBox1);
pictureBox1.Image = img;
pictureBox1.BackColor = Color.Transparent;
pictureBox1.Location = new Point(0, 0);
webBrowser1.Controls.Add(pictureBox2);
pictureBox2.Image = img;
pictureBox2.BackColor = Color.Transparent;
pictureBox2.Location = new Point(0, 0);
}
Image creategraphics(string text)
{
if (String.IsNullOrWhiteSpace(text)) return null;
var size = getWatermarkSize(text, WatermarkAngle, WatermarkFont, out double hypotenuse);
var img = new Bitmap(size.Width, size.Height);
var g = Graphics.FromImage(img);
var brush = new SolidBrush(WatermarkColor);
var stringFormat = new StringFormat() { Alignment = StringAlignment.Center, LineAlignment = StringAlignment.Center };
g.RotateTransform((float)-WatermarkAngle);
g.DrawString(text, WatermarkFont, brush, new Point(0, (int)hypotenuse / 2), stringFormat);
return img;
}
private static Size getWatermarkSize(string text, int angle, Font font, out double hypotenuse)
{
if (angle > 90) throw new ArgumentOutOfRangeException("angle");
using (var bitmap = new Bitmap(1, 1))
using (var graphics = Graphics.FromImage(bitmap))
{
var size = graphics.MeasureString(text, font);
hypotenuse = size.Width + size.Height;
double radians = (angle / 180.0) * Math.PI;
var width = (Math.Cos(radians) * size.Width) + (Math.Sin(radians) * size.Height);
var height = (Math.Cos(radians) * size.Height) + (Math.Sin(radians) * size.Width);
return new Size((int)width, (int)height);
}
}
}
This is what I get