The check carrying most of the weight is nine lines long. `resolves()` takes a site-absolute URL, drops everything after `#` or `?`, and treats a bare `/` as `dist/index.html` [9]. Then it does two filesystem tests: is there a file at DIST plus that path, and if not, is there an `index.html` inside a directory at that path [9]. The second fallback is why it works on a static build, where `/about` on the wire is `dist/about/index.html` on disk.
That is also the condition for any of this transferring. `existsSync` only tells the truth about your URLs if your URLs are your file paths. Put a server-side router, a rewrite rule, or a redirect map in front of `dist/` and the verifier starts reporting dead links that resolve fine in production, and missing ones that do not.
Read the extraction pattern as a scope statement. It is `/href="(\/[^"#][^"]*)"/g` [10], so a link is tested only when its first character is a slash and its second is neither a hash nor a quote; external and relative hrefs are never examined [11]. A protocol-relative href beginning `//` does clear the pattern, gets joined onto DIST, and fails as a missing file [11]. Cheap to special-case. Worth knowing before you lift the file into a repo with a CDN host in it.
The arithmetic behind the whole design: 200 lines out per minute, 200 lines back in per fifteen to twenty [1][2]. That is ten to thirteen lines a minute read properly, against a generator running fifteen to twenty times faster [3]. No amount of discipline closes a 15x gap, and the author says as much, having spent months recognising the shape of diffs and merging them [24].
The most useful number here is the one from the abandoned version. Writing the exhaustive rulebook up front produced forty rules, six of which ever fired [13] for a 15% hit rate [14], with the other thirty-four noisy enough that you stop reading the output [13]. A verifier nobody reads has a 100% pass rate. The author's framing is sharper: worse than no verifier, because now you believe you have one [15].
The IndexNow rule is the class of defect an agent cannot see. The key must match `/^[a-f0-9]{8,128}$/` and the matching file must be live at the site root or crawlers reject the submission [16]. Ask for IndexNow submission and you get a correct submission script and no key file, because the request never mentioned one, and the missing file fails silently by design [17]. `app-ads.txt` gets the same treatment, parsed field by field at build time against the IAB spec [18]. Knowledge of what silence costs has to live in the build, because it is not in the prompt.
Then the budget: performance at 90, accessibility, best practices and SEO all at 100 [19]. Performance is the only threshold with slack [20], and the reason is in the same file: shared CI runners are noisy, so the gate re-measures rather than failing on one bad sample [23]. That is the difference between a blocking performance floor and an advisory one, and it is good engineering. It also earned its keep on something no stylesheet review would surface, the grey used for code comments failing WCAG contrast, fixed in one line at commit d64bce0 [22].
169 lines is about 85% of one of those two-hundred-line diffs [25]. You read it once.
