Reserved image dimensions: 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 image-size · category performance · can count towards the checklist score

• —The real layout shift this page causes. That is a field measurement, and it needs traffic.
• —Whether a CSS rule or aspect-ratio already reserves the space.

Set width and height on images, or reserve the space in CSS.

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.

• HTML response weight
• Server response time
• Synchronous head scripts
• Image loading strategy

All checks and their definitions as JSON