Yes, you can start viewing the video file while it is still being downloaded if there is enough data already received.
To be able to view the video while the downloading via web browser is in progress, check that Options > Settings > General > Open while downloading is enabled in the web browser settings. If disabled, the video file cannot be viewed until the file has been completely downloaded.
Note that the progressive downloading of video is not the same thing as the streaming of the video. When streaming, the video transmission is controlled by the streaming server and viewing of the video can start as soon as the video stream begins. In contrast, in the progressive downloading the video file is downloaded from a web server without any streaming control and the file is stored in the device.