Synchronous head scripts: what it measures

The check reads the response your server returns and records what it found. That observation is stored with the report, so reopening an old audit shows what was true on the day it ran rather than what the page says now. It is not a live measurement, and it is not a model’s opinion of your page.

Ruleset blocking-scripts · category performance · can count towards the checklist score

• —Whether the script is actually blocking in practice, which depends on where and how it is inserted.
• —The cost in milliseconds. That is a lab measurement.

Add async or defer, or move the script out of the head.

Then verify it yourself rather than trusting the next report: reload the page, view the source, and look at the thing the check described. A check you can falsify in ten seconds is the only kind worth having.

The free vitals checks this on one URL and needs no signup — the same rule, on the same page, with no credit spent. When you want the whole page assessed, an audit is covered by the trial credit.

• Reserved image dimensions
• HTML response weight
• Server response time
• Image loading strategy

All checks and their definitions as JSON