vastlint vs Test A Tag

Short answer

Test A Tag is a small, free, no-signup utility for adops: paste HTML, MRAID, VAST, VAST URI, VMAP, or VMAP URI into a single box, and it auto-detects the format and renders it in an isolated preview frame. It adds a local "Stash" for saving up to 20 creatives in the browser and a "Share" button that encodes the creative into a link so a colleague's copy opens pre-loaded. That's the entire feature set.

vastlint answers a different question. It checks whether the VAST XML itself complies with the IAB specification, rule by rule, across VAST 2.0 through 4.3, and it can walk a wrapper chain hop by hop instead of only showing the final rendered result. Test A Tag has no rule engine, no error or warning output, and no wrapper-chain view.

They're not really substitutes. Test A Tag is a fast, disposable preview and sharing tool. vastlint is a compliance checker. A tag can render fine in Test A Tag's preview frame and still violate the spec in ways that only show up against a stricter player or an SSAI stitcher downstream.

The fastest way to choose between them

• Use vastlint if you need to know whether the VAST XML complies with the spec, not just whether it renders.
• Use vastlint if you need to see every wrapper hop on its own, not just the final resolved creative.
• Use vastlint if you want rule IDs and fix guidance you can paste into a ticket or a PR check.
• Use Test A Tag if you just want to paste something (HTML, MRAID, VAST, or VMAP) and see it render, no signup, no setup.
• Use Test A Tag if you want to stash a handful of tags locally or send a colleague a pre-loaded link.
• Use both if the tag matters: validate the XML in vastlint first, then use Test A Tag for a quick throwaway render or to hand a link to someone else.

Why vastlint is the stronger starting point

vastlint starts from the spec, not the player. It validates VAST XML against rules derived from the published IAB VAST XSD schemas and normative prose, across VAST 2.0 through 4.3, and reports findings with a rule ID, severity, and fix guidance rather than a render-or-doesn't-render result.

It also separates concerns that a single preview box collapses together: test a live tag URL, inspect each wrapper hop independently, and validate the resolved XML on its own. A tag can look fine in an isolated preview frame while still being structurally invalid in ways that break a stricter SDK, an SSAI stitcher, or downstream reporting.

That's the practical split. vastlint is stronger on spec compliance, wrapper visibility, and output you can act on. Test A Tag is stronger as a zero-friction way to eyeball a creative.

The gaps vastlint closes

• Rule-level validation against a public, standards-derived methodology, not just a render-or-doesn't render result.
• Wrapper-chain inspection showing every hop, not a single collapsed preview.
• Coverage through VAST 4.3, with a documented approach for the parts no published IAB XSD covers directly.
• Findings with a rule ID, severity, and fix guidance instead of no error reporting at all.
• Output built for reuse: CLI, CI checks, npm packages, and an MCP server, not only a browser tab.

What Test A Tag is actually built for

Test A Tag's own description is direct about its scope: a service "designed to preview creatives and ad tags directly on various browsers," supporting HTML5, MRAID, and video tags (VAST and VMAP), relying on Google's IMA SDK for video. It's framed explicitly as a convenience tool built to make previewing creatives "a little easier for the already overworked adops professional," not as a validator.

The interface is a single textarea that auto-detects whatever you paste, a preview tab with a reload button (useful for tags with frequency caps that need a fresh ad call), and a Stash tab. The Stash is plain browser localStorage, capped at 20 entries, device-specific, and gone if the browser data is cleared. The Share button encodes the pasted creative into a URL so someone else's session opens with it pre-loaded.

There's no signup, no support, and by its own disclaimer, no warranty: it's a free side tool, not a supported product. That also means there's nothing to validate against; it's rendering what you gave it, not checking it against anything.

What Test A Tag is useful for

• A zero-setup way to paste a creative or tag and see it render, with automatic format detection across HTML, MRAID, VAST, and VMAP.
• Reloading a tag quickly to test frequency-capped or fresh-ad-call behavior.
• Keeping a short local list of frequently used test tags without an account.
• Sending a colleague a link that opens with the exact same creative pre-loaded, without them needing to paste anything.

Use them in this order

If the first thing you do with a new tag is paste it into a preview frame, a passing render can quietly stand in for a compliance check it never performed. That's fine for a disposable creative you're eyeballing once, but it's a bad habit for anything going into a live campaign, an SSAI break, or a partner integration.

The cleaner order is to validate the VAST XML in vastlint first, resolve any wrapper chain hop by hop, and fix whatever the rule engine flags. Once the tag is known to be spec-compliant, Test A Tag is a fine place to do a quick visual pass or hand a colleague a working link.

That ordering keeps "is this XML correct" and "does this render for me right now" as separate questions, which is the point: one is a property of the document, the other is a property of a specific preview session.

When vastlint is the better first choice

• You need to know if the tag is spec-compliant, not just whether it renders in one preview frame.
• You need wrapper-chain visibility instead of a single collapsed preview result.
• You need rule IDs, severity, and fix guidance you can act on or paste into a ticket.
• You need output that plugs into CI, a CLI, or an agent workflow instead of a manual browser tab.
• The tag is going into a real campaign, not just being eyeballed once.

When Test A Tag still earns a spot

Test A Tag is genuinely useful when you want the lowest-friction way to glance at a creative, reload it a few times, or send someone a working link without asking them to paste anything. Its own scope is narrow by design: preview, stash, share. It doesn't claim to check anything against the IAB VAST specification, and it has no error or warning output to speak of.

That's the fair comparison. Test A Tag built a fast preview tool. vastlint built a validator for the spec. Use the second one first, and keep the first one around for the quick look afterward.