HTTP/3 on 27 homepages: all 27 answer over HTTP/2, and 12 advertise the next one

FIELD TEST · 2026-08-29 · 27 homepages · one request each · protocol negotiation and Alt-Svc headers

Sample and method: the same 30-site panel in use since 2026-08-15, each homepage requested once on 2026-08-29 with curl 8.7.1, a desktop browser user agent and redirects followed. Two values were recorded per site: the HTTP version the connection actually negotiated over TLS ALPN, and the Alt-Svc response header, which is how a server advertises that it also speaks something newer.

Every site on the panel answered over HTTP/2 — 27 of 27, with no HTTP/1.1 left anywhere. Twelve of those 27 advertise http/3 in an Alt-Svc header, so a browser would upgrade on the next visit. Google's crawler documentation lists two protocol versions it supports, and HTTP/3 is not one of them.

How we measured it

One request per homepage, redirects followed, headers kept. The negotiated version comes from curl's own report of the connection, which is the version agreed during the TLS handshake, so it reflects what the origin offered rather than what any intermediary preferred.

Two caveats belong here rather than at the end. The requests left this machine through a local HTTP CONNECT proxy with an exit in Osaka, which is why stripe.com resolved to /jp and slack.com to /intl/ja-jp/. And a CONNECT tunnel carries TCP, so no HTTP/3 request could be attempted at all — everything below about http/3 is what servers say they support, not what we made them do.

HTTP/2 is now the floor, not the upgrade

There is nothing to rank here. All 27 sites negotiated HTTP/2, which makes the interesting column the one where they differ.

Google's crawling documentation is direct about what this buys you with Googlebot, and the sentence is worth having in full: "crawling over HTTP/2 may save computing resources (for example, CPU, RAM) for your site and Googlebot, but otherwise there's no Google-product specific benefit to the site (for example, no ranking boost in Google Search)". The same page adds that "the default protocol version used by Google's crawlers is HTTP/1.1", and that the crawlers "will use the protocol version that provides the best crawling performance and may switch protocols between crawling sessions".

Your server offering HTTP/2 and Googlebot using it are two different facts, and only the first one is under your control.

The twelve that advertise http/3

They are: linear.app, figma.com, slack.com, discord.com, www.framer.com, webflow.com, www.shopify.com, www.cloudflare.com, substack.com, www.wired.com, www.theverge.com and www.bbc.com.

The header is short and the differences inside it are the story. Ten of the twelve set ma=86400, one day; two set ma=2592000, thirty days — slack.com and www.framer.com. That value is how long a client may keep using the alternative before re-checking, so a thirty-day cache is a much stronger commitment to the endpoint staying put.

Three sites still advertise pre-standard tokens alongside h3. www.bbc.com and www.theverge.com both offer h3-29 and h3-27, two draft numbers from before the specification was finished, and slack.com offers h3-29 together with quic, the older label from before the protocol got its final name. Nothing is broken about that — clients pick a token they know — but it is a header nobody has revisited in years.

The CDN decides this more than you do

Seven sites on the panel send server: cloudflare. Six of them advertise h3; www.notion.com is the one that does not, on the same infrastructure as six that do.

The other direction is just as clean. Three sites report server: Vercel — nextjs.org, vercel.com and supabase.com — and not one of them sends an Alt-Svc header. Five sites send no server header at all. So the column that looks like a technology choice is mostly a hosting choice, and for most site owners it is a setting they did not set.

What this means for you

Two moves, and the second one is smaller than it sounds.

1. Check which version your own site negotiates before optimising anything else about delivery. If it is still HTTP/1.1 you are in a minority of zero on this panel, and the fix is at your CDN or your server config, not in your pages.
2. Leave http/3 to your host. It is advertised, not configured, by most of the sites that have it, and Google's own crawlers do not list it among the protocols they support — so the audience for the upgrade is browsers.

One more line from the same Google page is worth knowing because it is the only opt-out documented: "To opt out from crawling over HTTP/2, instruct the server that's hosting your site to respond with a 421 HTTP status code when Google attempts to access your site over HTTP/2." If you have a proxy doing something creative with 421, that is worth checking. On compression the same page lists what the crawlers accept — "gzip, deflate, and Brotli (br)" — which is a cheaper win than any protocol version. The speed question this all hangs off is answered in does page speed affect SEO, and the timing side of the same panel is in time to first byte on 26 homepages. Whether a crawler gets anything useful once connected is what QueryWin works on.

What this run cannot tell you

Whether any of these servers can actually complete an HTTP/3 request. We could not attempt one, because the tunnel we go out through carries TCP and QUIC needs UDP, so a site advertising h3 here is making a claim we did not verify. We also measured one request per site at one moment from one place, which means a server that negotiates differently by region or by load looks like a single fact here. And none of this touches page speed: no timing, no byte counts, no rendering. A protocol version is not a performance measurement.

Common questions

How did you measure this?

One curl request per homepage on 2026-08-29, redirects followed, reading the negotiated HTTP version from the connection and the Alt-Svc header from the response. Both values come from the same request.

Does HTTP/2 or http/3 help SEO?

Google's documentation says crawling over HTTP/2 may save computing resources for both sides but gives "no ranking boost in Google Search". HTTP/3 is not listed among the versions its crawlers support at all.

What is the Alt-Svc header?

A response header where a server says the same content is also reachable over another protocol at another endpoint, with an ma value giving how long that may be remembered. Advertising it is how HTTP/3 gets used without a separate address.

Should I turn on HTTP/3?

If your CDN offers it as a switch, it costs nothing to enable and this panel says 12 of 27 comparable sites have it on. Nothing measured here shows a benefit, so treat it as housekeeping.

Why do some sites advertise h3-29 and h3-27?

Those are draft versions from before HTTP/3 was finalised, kept for clients that only know the older tokens. Three sites here still send them.