With the BetterFullscreen app extension, your iOS device will automatically enter full-screen mode when you rotate it while playing a video in Safari.

The extension has specific behaviours for different rotations:
• Landscape to portrait: Full-screen mode is requested only if the video will fill the entire portrait display without black bars.
• Portrait to landscape: Full-screen mode is always requested.
For all rotations: A full-screen request will not occur if the video resolution is below 144p or if the video has not been interacted with or started.

You can also customize the extension's behaviour in its settings menu, accessed in the Safari pop-up:
• Control for paused or muted video: By default, BetterFullscreen requests full-screen mode even if the video is paused or muted. Tap the slider next to the option to disable or re-enable this feature.
• Automatic unmuting: The extension can automatically unmute a video when it enters full-screen mode. This option is disabled by default to prevent conflicts with video controls on some websites.