Some users of the site I manage were reporting a browser error page that had no error code at all. After researching, I discovered through devtools that Cloudflare was returning status 503 on prefetch requests with the header “cf-speculation-refused: prefetch refused: not eligible.” The site uses Vue SPA. I have no concrete evidence, but I assume Speed Brain was causing this. I disabled it and will see if this behavior stops.
That is Speed Brain / Speculation indeed, but it shouldn’t be the cause of a browser error page. It’s expected that for pages not in cache, it returns a 503 and Chrome handles it fine/doesn’t preload it. If you were on an error page, and then hovered over links/etc, it’d speculate regardless of the error page.
Would this be valid for mobile browsers? as this is where users have reported issues
You’d have to include more info about the error.
If your question is about the support of speculation, then other then the fact that 503’s from it are expected, only specific mobile browsers support speculation / speed brain: <script type="speculationrules"> - HTML: HyperText Markup Language | MDN
Nothing on iOS, Samsung Internet/Opera Android/Chrome Android do though