Subtitle Size Adjuster

WHAT IT DOES

Subtitle Size Adjuster lets you change the font size of video subtitles and captions on any webpage. Use the slider to increase or decrease subtitle size from 25% to 400% of the default size, making captions easier to read. The tool supports WebVTT, TTML, and embedded caption tracks found in most HTML5 video players. Choose from preset sizes (75%, 100%, 125%, 150%, 200%) or add your own custom presets. Your preferred size is remembered per site, so subtitles are always the right size when you return to a site. A live preview shows the current size percentage, and changes apply instantly to all videos on the page.

WHERE IT RUNS

The extension runs entirely within your browser's local environment using the BotGentz framework. All subtitle detection, CSS injection, and size adjustments occur on your machine without any network requests. No video data, subtitle content, or user activity is ever transmitted, uploaded, or stored externally. The tool works on any website and respects the browser's security boundaries, operating within the isolated extension sandbox. Subtitle styling uses the native CSS ::cue pseudo-element and inline style overrides.

HOW TO USE

Click the extension toolbar icon to open the Subtitle Size Adjuster panel. Use the slider to adjust subtitle size—changes apply instantly to all videos on the page. The current size is displayed prominently at the top of the panel. Click preset buttons (75%, 100%, 125%, 150%, 200%) for quick size changes. Enter a custom size in the number input field (25-400). Add your own presets by entering a value and clicking "Add". Toggle "Show preview" to control whether the size display updates. Toggle "Remember per site" to save your preferred size for each domain. Set a "Default size" that applies to sites without a saved preference. The tool works automatically—when you visit a site, your saved subtitle size is applied.

THE SUBTITLE SIZE ENGINE — WHY IT WORKS

Most subtitle size adjusters rely on site-specific selectors that break when websites change. This tool uses a multi-layered approach that works across different video players and subtitle implementations. The primary method injects CSS rules using the ::cue pseudo-element, which is the standard way to style WebVTT captions across browsers. The CSS rule ::cue { font-size: X% !important; } scales all caption text proportionally. For players that don't use standard WebVTT, the tool also targets common subtitle container classes (.captions-text, .vtt-cue, .subtitle-text, etc.) and applies inline font-size overrides. The tool also iterates through video.textTracks to refresh cue rendering when size changes. The size is stored per domain using GM_setValue, with the domain as the key. This combined approach—CSS pseudo-element styling, class-based overrides, and textTrack manipulation—ensures subtitles are resized reliably across the widest range of video players.

THE PANEL

The Subtitle Size Adjuster panel is fully draggable by its title bar and remembers its position on screen per website. It snaps to the edges of your browser window for clean alignment. Collapse the panel to a compact icon strip using the minimize button, restoring it with a single click. Resize the panel by dragging any edge or corner; the size controls and presets adjust proportionally. Press the Escape key to close the panel instantly. The panel maintains independent state for each browser tab, allowing you to adjust subtitle sizes in multiple tabs simultaneously.

PLEASE NOTE

This extension requires the free BotGentz extension framework to be installed and enabled. The tool works with HTML5 video subtitles; embedded subtitles from platforms like YouTube may have platform-specific styling limitations. Some custom video players may not use standard caption elements and may not respond to size adjustments. The tool applies styles to all videos on the page; if a video has no subtitle track, no change will be visible. All subtitle size data is processed locally and is never transmitted, logged, or shared. The extension requests no network permissions and functions entirely offline. The source code is fully auditable and available for review.