--- title: "Unattributed Navigation Overhead (UNO) Is Underrated – CSS Wizardry" url: https://daily.dev/posts/unattributed-navigation-overhead-uno-is-underrated-css-wizardry-7pihq86rv source_url: https://csswizardry.com/2026/08/uno-is-underrated type: article source: "CSS Wizardry" author: "Harry Roberts" published: 2026-08-19T13:24:05.498Z updated: 2026-08-24T09:00:39.541Z tags: ["webdev", "google-chrome"] reading_time: 11 upvotes: 3 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Unattributed Navigation Overhead (UNO) Is Underrated – CSS Wizardry **[CSS Wizardry](https://daily.dev/sources/csswizardry)** · [@csswizardry](https://daily.dev/csswizardry) · 11 min read · 3 upvotes · 0 comments ## Summary Unattributed Navigation Overhead (UNO), a concept coined by Tim Vereecke, is the portion of Time to First Byte left over after subtracting the redirect, DNS, connection, and request-response phases reported by the Navigation Timing API. Because cross-origin redirects reset redirect timing to zero, hidden redirects (tracking links, shorteners, http-to-https jumps) inflate TTFB invisibly. A real-world SpeedCurve dataset showed 7.1 million UNO observations versus only 166 detectable redirects, illustrating how much navigation time goes unexplained. Chrome 151 introduces a Timing-Allow-Origin opt-in that lets destination origins see cross-origin redirect timing, but this only partially addresses the problem since not all providers will adopt it and UNO includes other browser-side delays too. The piece recommends tracking UNO as a first-class RUM metric alongside TTFB and its components. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What is Unattributed Navigation Overhead (UNO) in web performance monitoring? Unattributed Navigation Overhead is the portion of Time to First Byte remaining after subtracting the redirect, DNS, connection, and request-to-response phases exposed by the Navigation Timing API. It represents real time users experienced that named timing phases cannot explain, calculated as UNO = TTFB − redirect − DNS − connection − request-to-response-start. _daily.dev surfaces performance engineering deep dives for teams chasing down unexplained TTFB regressions._ ### Why does Navigation Timing show zero redirects even though a cross-origin redirect happened? For privacy reasons, the Navigation Timing API sets redirectStart, redirectEnd, and redirectCount to zero whenever a redirect crosses an origin boundary, even though the overall TTFB still includes that redirect's time. This affects routine cases like tracking links, shortened URLs, social wrappers, and even http-to-https redirects on the same hostname, since a scheme change counts as cross-origin. _Developers debugging hidden TTFB costs can track browser API nuances like this via daily.dev._ ### What did Chrome 151 change about measuring cross-origin redirect timing? Chrome 151 began rolling out a Timing-Allow-Origin opt-in that lets redirecting servers permit the destination origin to measure redirects under their control, something previously impossible for navigation redirects. It does not eliminate the need for tracking Unattributed Navigation Overhead, since every server in a chain must opt in, many shorteners and campaign platforms won't add the header, and other browsers may not support it yet. _Keep up with browser API changes like this on daily.dev before they affect your performance instrumentation._ ## Similar posts on daily.dev - [Web-Perf Wednesday 002 – The Metrics Don’t Tell the Whole Story – CSS Wizardry](https://daily.dev/posts/web-perf-wednesday-002-the-metrics-don-t-tell-the-whole-story-css-wizardry-757w9utar) · CSS Wizardry · 4 upvotes · 0 comments - [Introducing LUX Sidecar: Your SpeedCurve Companion – CSS Wizardry](https://daily.dev/posts/introducing-lux-sidecar-your-speedcurve-companion-css-wizardry-tdkjc6uss) · CSS Wizardry · 4 upvotes · 0 comments - [Insights from 100 site speed reviews in 2025](https://daily.dev/posts/insights-from-100-site-speed-reviews-in-2025-yh4tk3cne) · Web Performance Calendar · 0 upvotes · 0 comments --- Tags: [#webdev](https://daily.dev/tags/webdev), [#google-chrome](https://daily.dev/tags/google-chrome) [View this post on daily.dev](https://daily.dev/posts/unattributed-navigation-overhead-uno-is-underrated-css-wizardry-7pihq86rv)