diff --git a/dotcom-rendering/src/components/SelfHostedVideo.island.tsx b/dotcom-rendering/src/components/SelfHostedVideo.island.tsx
index 22cbc3e385b..d51df1b1fbb 100644
--- a/dotcom-rendering/src/components/SelfHostedVideo.island.tsx
+++ b/dotcom-rendering/src/components/SelfHostedVideo.island.tsx
@@ -21,6 +21,7 @@ import {
 	customSelfHostedVideoPlayAudioEventName,
 	customYoutubePlayEventName,
 	findOptimisedSourcePerMimeType,
+	getVideoStrategy,
 } from '../lib/video';
 import type { VideoStyleSettings } from '../lib/videoStyleSettings';
 import { videoSettingsMap } from '../lib/videoStyleSettings';
@@ -31,6 +32,7 @@ import type { RenderingTarget } from '../types/renderingTarget';
 import { Caption } from './Caption';
 import { CardPicture, type Props as CardPictureProps } from './CardPicture';
 import { useConfig } from './ConfigContext';
+import { SelfHostedVideoDebugOverlay } from './SelfHostedVideoDebugOverlay';
 import type {
 	ControlsPosition,
 	PLAYER_STATES,
@@ -144,6 +146,7 @@ const fullscreenStyles = css`
 
 		/* Override the fixed aspect-ratio + width:100% on the video so it
 		   fits within the screen while preserving its aspect ratio. */
+
 		video {
 			width: 100%;
 			height: 100%;
@@ -650,10 +653,13 @@ export const SelfHostedVideo = ({
 		}
 
 		const screenWidth = window.innerWidth;
+		const strategy = getVideoStrategy();
+		console.log(strategy);
 		const filteredSources = findOptimisedSourcePerMimeType(
 			sources,
 			screenWidth,
-		);
+		).filter((source) => source.mimeType === strategy);
+
 		setOptimisedSources(filteredSources);
 
 		/**
@@ -1137,6 +1143,11 @@ export const SelfHostedVideo = ({
 						isInteractive={videoStyleSettings.isInteractive}
 						isWebKitFullscreen={isWebKitFullscreen}
 					/>
+					<SelfHostedVideoDebugOverlay
+						videoRef={vidRef}
+						atomId={atomId}
+						renderingTarget={renderingTarget}
+					/>
 				</div>
 			</div>
 			{!!caption && format && (
diff --git a/dotcom-rendering/src/components/SelfHostedVideoDebugOverlay.tsx b/dotcom-rendering/src/components/SelfHostedVideoDebugOverlay.tsx
new file mode 100644
index 00000000000..7d6e220b849
--- /dev/null
+++ b/dotcom-rendering/src/components/SelfHostedVideoDebugOverlay.tsx
@@ -0,0 +1,315 @@
+import { css } from '@emotion/react';
+import { headlineMedium14 } from '@guardian/source/foundations';
+import { useEffect, useRef, useState } from 'react';
+import { palette } from '../palette';
+import type { RenderingTarget } from '../types/renderingTarget';
+
+/**
+ * A lightweight, intentionally disposable debug overlay for self-hosted video.
+ *
+ * Enable by appending `?videoDebug=1` (or `&videoDebug=1`) to the page URL.
+ *
+ * Surfaces:
+ *  - FORMAT:   mime/extension of the selected source
+ *  - STARTUP:  ms from `loadstart` to first `playing`
+ *  - STALLS:   count of `waiting`/`stalled` events after initial play
+ *  - CACHE:    HIT / MISS / UNKNOWN, derived from Resource Timing
+ *  - NETWORK:  navigator.connection.effectiveType (4g, 3g, …)
+ *  - RUNTIME:  WKWebView / Android WebView / Safari / Chrome / Firefox / …
+ *  - SIZE:     intrinsic video dimensions & aspect ratio
+ *  - STATE:    current readyState / paused / muted
+ */
+
+type Props = {
+	videoRef: React.RefObject<HTMLVideoElement>;
+	atomId: string;
+	renderingTarget: RenderingTarget;
+};
+
+const overlayStyles = css`
+	position: absolute;
+	top: 8px;
+	left: 8px;
+	z-index: 2147483647;
+	padding: 8px 10px;
+	background: ${palette('--versus-change')};
+	color: ${palette('--uk-elections-scottish-national-party')};
+	${headlineMedium14};
+	white-space: pre;
+	pointer-events: none;
+	border-radius: 4px;
+	max-width: calc(100% - 16px);
+	overflow: hidden;
+	text-shadow: 0 0 2px ${palette('--versus-change')};
+`;
+
+const detectRuntime = (renderingTarget: RenderingTarget): string => {
+	if (typeof navigator === 'undefined') return 'SSR';
+	const ua = navigator.userAgent;
+
+	// Guardian apps render in a native WebView
+	if (renderingTarget === 'Apps') {
+		if (/iPhone|iPad|iPod/i.test(ua)) return 'WKWebView';
+		if (/Android/i.test(ua)) return 'Android WebView';
+		return 'App WebView';
+	}
+
+	// iOS detection
+	if (/iPhone|iPad|iPod/i.test(ua)) {
+		const isStandalone =
+			'standalone' in navigator &&
+			(navigator as Navigator & { standalone?: boolean }).standalone ===
+				true;
+		if (isStandalone) return 'WKWebView (standalone)';
+		if (/CriOS/i.test(ua)) return 'Chrome iOS';
+		if (/FxiOS/i.test(ua)) return 'Firefox iOS';
+		if (/Safari/i.test(ua)) return 'Safari iOS';
+		return 'WKWebView';
+	}
+
+	if (/Edg\//i.test(ua)) return 'Edge';
+	if (/OPR\//i.test(ua)) return 'Opera';
+	if (/Firefox/i.test(ua)) return 'Firefox';
+	if (/Chrome/i.test(ua)) return 'Chrome';
+	if (/Safari/i.test(ua)) return 'Safari';
+	return 'Unknown';
+};
+
+type ConnectionLike = {
+	effectiveType?: string;
+	downlink?: number;
+	rtt?: number;
+	saveData?: boolean;
+};
+
+const getConnection = (): ConnectionLike | undefined => {
+	if (typeof navigator === 'undefined') return undefined;
+	return (navigator as Navigator & { connection?: ConnectionLike })
+		.connection;
+};
+
+const getCacheStatus = (src: string): 'HIT' | 'MISS' | 'UNKNOWN' => {
+	if (!src || typeof performance === 'undefined') return 'UNKNOWN';
+	const entries = performance.getEntriesByName(
+		src,
+	) as PerformanceResourceTiming[];
+	if (entries.length === 0) return 'UNKNOWN';
+	const entry = entries[entries.length - 1];
+	if (!entry) return 'UNKNOWN';
+
+	// transferSize === 0 typically means a memory/disk cache hit
+	// (but can also mean a cross-origin response without TAO header).
+	if (entry.transferSize === 0 && entry.decodedBodySize > 0) return 'HIT';
+	// Heuristic: very fast response start ⇒ likely cache
+	if (entry.responseStart - entry.requestStart < 5) return 'HIT';
+	return 'MISS';
+};
+
+const getFormat = (video: HTMLVideoElement | null): string => {
+	if (!video?.currentSrc) return '—';
+	const src = video.currentSrc;
+	const ext = src.split('?')[0]?.split('#')[0]?.split('.').pop();
+	if (!ext) return 'unknown';
+	if (ext.toLowerCase() === 'm3u8') return 'HLS';
+	return ext.toUpperCase();
+};
+
+const useQueryParam = (name: string): string | null => {
+	const [value, setValue] = useState<string | null>(null);
+	useEffect(() => {
+		if (typeof window === 'undefined') return;
+		const params = new URLSearchParams(window.location.search);
+		setValue(params.get(name));
+	}, [name]);
+	return value;
+};
+
+export const SelfHostedVideoDebugOverlay = ({
+	videoRef,
+	atomId,
+	renderingTarget,
+}: Props) => {
+	const enabled = useQueryParam('videoDebug');
+	const [, force] = useState(0);
+
+	const loadStartRef = useRef<number | null>(null);
+	const startupRef = useRef<number | null>(null);
+	const stallsRef = useRef(0);
+	const hasPlayedRef = useRef(false);
+
+	useEffect(() => {
+		if (enabled !== '1') return;
+		const video = videoRef.current;
+		if (!video) return;
+
+		const rerender = () => force((n) => n + 1);
+
+		// iOS Safari often fires `loadstart` before this effect attaches its
+		// listener (the overlay mounts after the <video> element). In that
+		// case, seed loadStartRef from the resource timing entry's startTime
+		// so STARTUP can still be computed when `playing` / `timeupdate`
+		// eventually fires. Fall back to "now" if no entry is available.
+		const seedLoadStartFromResourceTiming = () => {
+			if (loadStartRef.current !== null) return;
+			const src = video.currentSrc;
+			if (src && typeof performance !== 'undefined') {
+				const entries = performance.getEntriesByName(
+					src,
+				) as PerformanceResourceTiming[];
+				const entry = entries[entries.length - 1];
+				if (entry) {
+					loadStartRef.current = entry.startTime;
+					return;
+				}
+			}
+			loadStartRef.current = performance.now();
+		};
+
+		const computeStartup = () => {
+			if (startupRef.current !== null) return;
+			if (loadStartRef.current === null) {
+				seedLoadStartFromResourceTiming();
+			}
+			if (loadStartRef.current !== null) {
+				startupRef.current = Math.max(
+					0,
+					Math.round(performance.now() - loadStartRef.current),
+				);
+			}
+		};
+
+		const onLoadStart = () => {
+			loadStartRef.current = performance.now();
+			startupRef.current = null;
+			stallsRef.current = 0;
+			hasPlayedRef.current = false;
+			rerender();
+		};
+		const onPlaying = () => {
+			if (!hasPlayedRef.current) {
+				computeStartup();
+			}
+			hasPlayedRef.current = true;
+			rerender();
+		};
+		// iOS Safari is unreliable about firing `playing`; the first
+		// `timeupdate` with currentTime > 0 is a robust fallback.
+		const onTimeUpdate = () => {
+			if (!hasPlayedRef.current && video.currentTime > 0) {
+				computeStartup();
+				hasPlayedRef.current = true;
+				rerender();
+			}
+		};
+		const onStallish = () => {
+			if (hasPlayedRef.current) {
+				stallsRef.current += 1;
+				rerender();
+			}
+		};
+		const passthrough = () => rerender();
+
+		video.addEventListener('loadstart', onLoadStart);
+		video.addEventListener('playing', onPlaying);
+		video.addEventListener('timeupdate', onTimeUpdate);
+		video.addEventListener('waiting', onStallish);
+		video.addEventListener('stalled', onStallish);
+		video.addEventListener('loadedmetadata', passthrough);
+		video.addEventListener('canplay', passthrough);
+		video.addEventListener('pause', passthrough);
+		video.addEventListener('ended', passthrough);
+		video.addEventListener('ratechange', passthrough);
+		video.addEventListener('volumechange', passthrough);
+
+		// If the video already started loading (or is already playing) before
+		// we attached listeners — common on iOS Safari — back-fill state now.
+		if (video.currentSrc || video.readyState > 0) {
+			seedLoadStartFromResourceTiming();
+		}
+		if (!video.paused && video.currentTime > 0) {
+			computeStartup();
+			hasPlayedRef.current = true;
+		}
+
+		// Re-render every second so connection / cache snapshots stay fresh
+		const interval = window.setInterval(rerender, 1000);
+
+		return () => {
+			video.removeEventListener('loadstart', onLoadStart);
+			video.removeEventListener('playing', onPlaying);
+			video.removeEventListener('timeupdate', onTimeUpdate);
+			video.removeEventListener('waiting', onStallish);
+			video.removeEventListener('stalled', onStallish);
+			video.removeEventListener('loadedmetadata', passthrough);
+			video.removeEventListener('canplay', passthrough);
+			video.removeEventListener('pause', passthrough);
+			video.removeEventListener('ended', passthrough);
+			video.removeEventListener('ratechange', passthrough);
+			video.removeEventListener('volumechange', passthrough);
+			window.clearInterval(interval);
+		};
+	}, [enabled, videoRef]);
+
+	if (enabled !== '1') return null;
+
+	const video = videoRef.current;
+	const connection = getConnection();
+	const runtime = detectRuntime(renderingTarget);
+	const format = getFormat(video);
+	const cache = video?.currentSrc
+		? getCacheStatus(video.currentSrc)
+		: 'UNKNOWN';
+	const startup =
+		startupRef.current !== null ? `${startupRef.current}ms` : '—';
+	const stalls = stallsRef.current;
+	const network = connection?.effectiveType ?? 'unknown';
+	const downlink =
+		connection?.downlink !== undefined ? `${connection.downlink}Mbps` : '—';
+	const rtt = connection?.rtt !== undefined ? `${connection.rtt}ms` : '—';
+	const saveData = connection?.saveData ? ' (saveData)' : '';
+
+	const size = video
+		? `${video.videoWidth}x${video.videoHeight} (${
+				video.videoHeight > 0
+					? (video.videoWidth / video.videoHeight).toFixed(2)
+					: '—'
+		  })`
+		: '—';
+
+	const readyState = (() => {
+		switch (video?.readyState) {
+			case 0:
+				return 'HAVE_NOTHING';
+			case 1:
+				return 'HAVE_METADATA';
+			case 2:
+				return 'HAVE_CURRENT_DATA';
+			case 3:
+				return 'HAVE_FUTURE_DATA';
+			case 4:
+				return 'HAVE_ENOUGH_DATA';
+			default:
+				return '—';
+		}
+	})();
+
+	const state = video
+		? `${readyState}${video.paused ? ' · paused' : ' · playing'}${
+				video.muted ? ' · muted' : ''
+		  }`
+		: '—';
+
+	const lines = [
+		`ATOM:    ${atomId}`,
+		`FORMAT:  ${format}`,
+		`STARTUP: ${startup}`,
+		`STALLS:  ${stalls}`,
+		`CACHE:   ${cache}`,
+		`NETWORK: ${network} · ${downlink} · rtt ${rtt}${saveData}`,
+		`RUNTIME: ${runtime}`,
+		`SIZE:    ${size}`,
+		`STATE:   ${state}`,
+	];
+
+	return <div css={overlayStyles}>{lines.join('\n')}</div>;
+};
diff --git a/dotcom-rendering/src/lib/video.ts b/dotcom-rendering/src/lib/video.ts
index 3ecdb55bf6a..4406efc23c8 100644
--- a/dotcom-rendering/src/lib/video.ts
+++ b/dotcom-rendering/src/lib/video.ts
@@ -177,3 +177,24 @@ export const formatTimeForDisplay = (timeInSeconds: number): string => {
 
 	return `${minutes}:${seconds.toString().padStart(2, '0')}`;
 };
+
+type VideoStrategy =
+	| 'video/mp4'
+	| 'application/vnd.apple.mpegurl'
+	| 'application/x-mpegURL';
+
+export const getVideoStrategy = (): VideoStrategy => {
+	const param = new URLSearchParams(location.search).get('videoStrategy');
+
+	switch (param) {
+		case 'mp4':
+			return 'video/mp4';
+		case 'hls':
+			return 'application/vnd.apple.mpegurl';
+
+		case 'hls-alt':
+			return 'application/x-mpegURL';
+		default:
+			return 'video/mp4';
+	}
+};