vastlint

VAST 4.3 validator: what to check in 4.3 tags

Short answer

VAST 4.3 is a small release, but it still has version-specific checks that break tooling when they are ignored. The main one vastlint calls out directly is browserOptional on <JavaScriptResource>, especially for CTV and audio environments with no browser runtime.

Why this happens

Teams often treat 4.3 as "basically 4.2" and skip the version-specific review. That is partly true, but not fully safe. VAST 4.3 added guidance around interactive and verification behavior that matters most in mixed browser and non-browser environments.

So the right 4.3 workflow is not just checking the root version string. It is validating the surrounding assets and their runtime assumptions.

Copyable fix example

<JavaScriptResource apiFramework="omid" browserOptional="false">
  <![CDATA[https://cdn.example.com/omid.js]]>
</JavaScriptResource>

Exact rule pages

Validate VAST 4.3 the right way

If you already have the XML, run the validator. If the 4.3 response is behind a live tag URL or wrapper chain, use the tester or inspector first so you can validate the exact payload the player sees.

Validate this XML →Test this live URL →Inspect this wrapper chain →

Further reading