C# Click a button in a web page opened with default browser

I have to open a web link in the default browser with my application (and so far I have succeeded) and click on a button. I tried with two methods 1. Setting the coordinates trying them with all the screen size (800x600 etc), but it is not very safe and reliable 2. Creating a bitmap screenshot and looking for the pixel with the button color, but it is too slow and unreliable

Is there another way to do it? Thanks