CheckSafe.Zone
for marketers
Specs last verified: May 2026 · About this tool
A clickTag is a JavaScript variable (typically window.clickTag or var clickTag) that ad servers inject at runtime to control where the banner links when clicked. Instead of hardcoding a destination URL inside the banner, the click destination is passed in by the trafficking platform — Google Campaign Manager 360, DV360, Xandr, The Trade Desk, and all major DSPs require it.
Without a clickTag, the banner cannot be made clickable by the ad server, which means it will either fail QC, default to a blank click, or be rejected outright. The Banner Validator scans all HTML and JS files inside the ZIP to detect any standard clickTag implementation.
The IAB Tech Lab recommends an initial load of 200 KB for display ads, but most DSPs and ad servers enforce a stricter 150 KB limit on the ZIP file. Google Campaign Manager 360 and DV360 enforce 150 KB as the hard limit for HTML5 creatives. Exceeding this causes the banner to fail upload or to serve a blank backup image instead.
The Banner Validator flags any file over 150 KB with a red warning chip. This covers the full ZIP size, which is what ad servers measure — not the size of individual assets inside it.
The validator recognises all major IAB Universal Ad Package and Rising Stars formats: Medium Rectangle (300×250), Leaderboard (728×90), Wide Skyscraper (160×600), Half Page (300×600), Mobile Banner (320×50), Large Mobile Banner (320×100), Billboard (970×250), Large Leaderboard (970×90), Banner (468×60), Square (250×250), Small Square (200×200), Skyscraper (120×600), Portrait (300×1050), and Vertical Rectangle (240×400).
If the dimensions don't match a known format, the pixel dimensions are shown without a label — useful for custom or non-standard placements.
No. Everything runs entirely inside your browser. HTML5 ZIP files are parsed in-browser using the JSZip library, GIF frames are decoded using binary parsing, and image dimensions are read using the native Canvas API — no file ever leaves your device or touches a server.
This makes the tool safe for agencies working under NDAs, pre-launch campaign creatives, or any situation where files must remain confidential. The tool also works offline once the page has loaded.
The most common causes: the ZIP does not contain an index.html at the root level (or inside a single subfolder), the banner makes external HTTP requests to load assets (these are blocked for security in the sandboxed preview), or the animation relies on a library loaded from a CDN rather than bundled inside the ZIP.
For the preview to work, all assets — images, fonts, JS libraries — should be bundled inside the ZIP. External CDN calls (e.g. loading GSAP or fonts from a remote URL) will fail in the sandboxed preview environment by design, since banner ad servers block external requests for the same reason.
A GIF banner is a single animated image file — simple, widely supported, but limited to 256 colours, larger file sizes, and no click interactivity beyond a single destination URL. Most modern placements cap GIFs at 150 KB and three animation loops.
An HTML5 banner is a ZIP package containing an HTML file, CSS, JavaScript, and assets. It supports full-colour animation (CSS, GSAP, or Canvas), dynamic content, multiple click destinations via clickTag, and backup image fallback for environments that block JavaScript. HTML5 is the current standard for premium display advertising across all major DSPs.
Before uploading to Google Campaign Manager 360 or Display & Video 360: confirm the ZIP is under 150 KB total, verify a clickTag variable is present and correctly implemented, ensure all assets are bundled inside the ZIP with no external URL dependencies, check the animation loops no more than three times and runs no longer than 30 seconds (IAB and Google policy), and confirm a static backup image (backup.jpg or backup.png) is included for environments where JavaScript is blocked.
The Banner Validator covers file size and clickTag detection automatically. Checking dimensions against your media plan's spec sheet before upload prevents rejection at the line item level.
A backup image (also called a fallback image) is a static JPEG or PNG file included inside the HTML5 ZIP — typically named backup.jpg, backup.png, fallback.jpg, or fallback.png — that ad servers serve instead of the animated HTML5 creative in environments where JavaScript is blocked. Google Campaign Manager 360 and DV360 require a backup image for all HTML5 creatives. Without one, the ad server may serve a blank space or reject the upload. The Banner Validator automatically detects and previews backup images found in your ZIP.
Google Campaign Manager 360 accepts HTML5 ZIP creatives under 150 KB (initial load), static images (JPG, PNG, GIF) at standard IAB dimensions, and animated GIFs with a maximum of three loops. For HTML5 banners: the ZIP must contain an index.html at the root level, include a clickTag variable, bundle all assets internally with no external CDN calls, and include a backup.jpg or backup.png. CM360 also enforces a 30-second maximum animation duration and requires HTTPS click destinations.
The IAB Universal Ad Package defines four core sizes: Medium Rectangle (300×250), Leaderboard (728×90), Wide Skyscraper (160×600), and Half Page (300×600). Additional common formats include Mobile Banner (320×50), Large Mobile Banner (320×100), Billboard (970×250), Large Leaderboard (970×90), Portrait (300×1050), and Banner (468×60). The Banner Validator automatically identifies all 14 standard IAB formats and displays the format name alongside pixel dimensions. Non-standard dimensions are shown as pixels only, without a format label.
The most common DV360 rejection causes: the ZIP file exceeds 150 KB, no clickTag variable is found or it is incorrectly implemented, external asset requests exist inside the HTML (Google blocks these for security), a backup image is missing, index.html is not at the ZIP root level, or the animation exceeds 30 seconds. Check the Banner Validator's chips for file size and clickTag status. If both pass, open the ZIP and verify that all fonts, images, and JS libraries are bundled inside — any src or href pointing to an external URL will cause DV360 to reject or sandbox the creative.
Most major ad networks — including Google Display Network, DV360, and The Trade Desk — cap GIF banner animations at three loops before the animation must come to rest on a final frame. IAB guidelines enforce this as a standard. Infinite-loop GIFs are typically rejected or served without animation on premium placements. The Banner Validator detects and displays the loop count for GIF files, flagging infinite loops so you can catch this before trafficking.
Initial load is the total size of all files that download when the ad first appears — this is the 150 KB limit enforced by CM360, DV360, and most DSPs. Subload (also called polite load or subsequent load) refers to additional assets that load after the host page has finished rendering, typically capped at 2.5 MB by IAB guidelines. The Banner Validator measures the full ZIP size, which represents the initial load. Assets loaded dynamically via JavaScript after render are not included in the ZIP and are not measured by this tool.
The Banner Validator works with any standard HTML5 ZIP that contains an index.html at the root level. Celtra-exported ZIPs are typically compatible when exported as a Standard HTML5 package. Smartly creatives exported as HTML5 ZIPs should also work, provided assets are bundled and not loaded from Smartly's CDN. If the preview shows blank, it is usually because the creative loads assets from an external URL — which is blocked in the sandboxed preview for the same reason ad servers block external requests.