What is measured
The audit analyses src and href values for images, scripts, styles, frames and media that use absolute http addresses.
Trust and structured data
Mixed content
This check finds HTTP resources inside an HTTPS page that browsers may block or load insecurely.
Pass criterion
Every active and visual resource loads over HTTPS or a safe relative URL.
What to fix
Replace resource addresses, update the CDN and remove dependencies on hosts without HTTPS.
Automation limit
Static HTML analysis may miss an address inserted by a third-party script only after browser execution.
Reference