Speculation Rules
You can read up on speculation rules at MDN. A summary for different types and eagerness levels are shown below.
Use this Speculation Rules generator to easily create a tailored set for your website.
Browser support
Speculation types
| Type | HTML Only | Subresources | Script Execution | Activation Cost |
|---|---|---|---|---|
| prefetch | ✅ | ❌ | ❌ | ✔️ low |
| full prerender | ✅ | ✅ | ✅ | ❗ high |
| prerender until script | ✅ | ✅ | ⛔ paused until nav | ⭐ medium |
Eagerness levels
| eagerness | When it triggers (roughly) | Speed boost | Waste risk | Best for |
|---|---|---|---|---|
| (default) | depends on rule type:urls → immediatewhere → conservative
|
⭐⭐ | ⭐ | “do the right thing” default |
immediate |
ASAP (subject to settings + resources) | ⭐⭐⭐⭐ | ⭐⭐⭐⭐ | very likely next nav / expensive pages |
eager |
early interest signals (hover-ish / prominent visibility) | ⭐⭐⭐ | ⭐⭐⭐ | aggressive UX optimization |
moderate |
stronger intent (in viewport + sustained hover/focus) | ⭐⭐ | ⭐⭐ | balanced performance vs cost |
conservative |
only at click initiation (pointerdown / mousedown) | ⭐ | ⭐ | minimize wasted work |