Media diagnostics · Primary-source explanation; examples are illustrative. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
Observe the video rather than the window
A developer may keep a player box fixed at the same size while the dimensions reported for the actual video change. MDN documents a resize event on HTMLVideoElement which fires when videoWidth, videoHeight or both have been updated. This is a useful observation point for an intrinsic media change. Listening only to the browser window would miss this distinction and could leave a diagnostic overlay displaying dimensions from an earlier part of playback.
Keep three sizes separate
Consider an explicitly hypothetical example: a page has a 960 by 540 CSS player box, the connected display is 3840 by 2160, and the video reports 1280 by 720 before later reporting 1920 by 1080. None of those three measurements has to equal another. Record them under separate labels. A larger CSS box can enlarge existing pixels, whereas a changed intrinsic video dimension is a different observation requiring its own timestamp.
Avoid an unsupported quality conclusion
The event establishes a dimension update; it does not alone establish why it happened, what bitrate was used, or whether the source was natively captured at that size. Pair the observation with the selected rendition information when the player legitimately exposes it. Keep any user chosen quality mode in the report. This is a method for describing a change accurately, not a claim that every UK IPTV programme switches resolution or that resolution alone defines picture quality.