Use the keybd_event API function to send the application Ctrl-A to select all of the text and then Ctrl-C to copy the text to the clipboard. Then use the Clipboard object's GetText method to get the text.

' Grab all of the text in the WebBrowser control.
Private Sub Command1_Click()
    ' Clear the clipboard.
    Clipboard.Clear

    ' Set focus to the WebBrowser control.
    WebBrowser1.SetFocus

    ' Control-A.
    ' Press Control.
    keybd_event VK_CONTROL, 0, 0, 0
    DoEvents

    ' Press A.
    keybd_event VK_A, 1, 0, 0
    DoEvents

    ' Control-C.
    ' Press Control.
    keybd_event VK_CONTROL, 0, 0, 0
    DoEvents

    ' Press C.
    keybd_event VK_C, 1, 0, 0
    DoEvents

    ' Release Control.
    keybd_event VK_CONTROL, 0, KEYEVENTF_KEYUP, 0
    DoEvents

    ' Get the text from the clipboard.
    Text1.Text = Clipboard.GetText
End Sub