If you use Safari, you can use this simple code:
tell application "Safari"
text of document 1
end tell
If you will use a different browser which doesn�t support such methods, you can use UI Scripting (System Events 1.2.1 is built-in in Panther, and there is a previous beta version available for Jaguar).
The concept is very simple: use the common menu items “Select all” and “Copy”, then pick your stuff from the clipboard. You may adjust this code depending on your browser:
tell application "System Events"
tell process "Safari"
set frontmost to true --> bring app to the foreground
click menu item 8 of menu 1 of menu bar item 4 of menu bar 1 --> select all
click menu item 5 of menu 1 of menu bar item 4 of menu bar 1 --> copy
end tell
end tell
set theText to the clipboard
You can find here some tools to help you developing UI scripts.