Declare the track's purpose
A file with WebVTT syntax may carry captions, chapters or metadata. A validator needs to know which type is intended before applying purpose-specific checks. An overlap report without that context is incomplete.
The W3C WebVTT Candidate Recommendation Draft of 20 May 2026 permits caption timing relationships that need different treatment from its chapter-title track model. Chapter structure is described through nested cues. Avoid turning that into the inaccurate shortcut that every chapter overlap is forbidden.
Compare containment with crossing
Consider an invented chapter from 0 to 60 seconds containing a subsection from 10 to 20. That containment can express hierarchy. A second chapter from 40 to 80 crosses the first boundary and needs a different structural review; neither contains the other.
For captions, two speakers talking at once can justify simultaneous cue intervals. The production question then includes legibility, attribution and placement, rather than automatically shortening one cue until overlap disappears. A parser accepting the file does not settle that editorial question.
Keep validation results specific
Report track type, cue identifiers and the exact timing relationship being flagged. Distinguish malformed timestamps, inappropriate hierarchy and visually colliding captions. Those findings lead to different fixes.
After editing chapter structure, check navigation labels against the final cut. After editing simultaneous captions, review the actual presentation at the chosen size. This is a format-based review method, not a compatibility result from any specific player.
Questions answered
Is every WebVTT overlap invalid?
No. The track's purpose and timing relationship matter.
Can chapter cues contain subsections?
The draft's chapter model uses nested cue relationships; check hierarchy rather than rejecting containment automatically.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.