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Preload, prefetch and other link tags (3perf.com)
157 points by iamakulov 36 days ago | hide | past | web | favorite | 26 comments



You can also put prefetch hints in a header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Link_prefe...


It's unfortunate that Chrome's definition of 'mandatory' is flexible in a bad way. If you've done enough performance profiling of webapps in Chrome you may have noticed that even cached resources can take over 20ms to load from cache - this is because all cache loads have to go between processes, so they get delayed until thread/process switches can happen and messages can get ferried back and forth. If you're unlucky, you eat this delay on every resource you load. In some cases this can add over a second to load time for an application.

You might think 'well this calls for prefetch, right? load them all at once'. You're right! Except it doesn't work. Chrome content processes ignore prefetch directives, all they seem to do is ensure that they're in cache - so there's no way to avoid eating this 20+ms hit per-resource.

Ideally you control all the code being run in your app so you can just issue the relevant XHR requests to load everything up-front and not block on those requests, but I've frequently run into cases where a downloaded resource loads other resources, etc. Really frustrating that caching and prefetch are so slow in Chrome and there's no indication that they will fix it.

It's my understanding that in comparison Safari does not have this hit per-asset, loading the applications I've tested is dramatically faster on an iPhone compared to Chrome on a desktop PC. :(


Random question: for the preconnect, is there some Javascript API now that allows for the monitoring of the TLS handshake?

As specified in the article, the handshake is:

   TLS handshake (only for HTTPS sites). Perform two roundtrips 
   (a message goes client → server → client → server → client) to initiate a secure TLS session
Is there any part of that which could be used to send a custom message to the client? A short bit of text or a number or something. Don't really have a practical use for this, just more of an esoteric one.


You can't do it from a browser, but the "tls client hello" could certainly hold a custom message of sorts. Either in the "random bytes" it's supposed to send, or stuffed in as a fake cipher preference in the supplied list.

Good breakdown of the handshake here: http://www.moserware.com/2009/06/first-few-milliseconds-of-h... (skip down to the client hello)


It’s silly o’clock here (unable to sleep) and the following is just going off the top of my head, apologises if I mess it up.

Don’t believe so as during the first round trip in http1 (wasn’t this “fixed” in http2? Silly o’clock, can’t remember, anyways) the client and the server are just working out which cyphers to use.

If you are trying to pull data from the same domain, you have already set up a connection you can reuse saving the need to repeat the need for the first round trip on your next connection to the server. So you are only really losing that first round trip on the first connection. So even if you were pinging another domain regularly as long as you are using a server configured to keep the connection alive for long enough you are not really going to save much any time then just doing it the standard way.

I guess (never tried) you could create your own raw socket connection with say webasm but Browsers do here hardest to keep you away from the raw socket level and I believe they also do even in webasm. (just thinking out loud...)

Then smuggle in your value based on the the cyphers you are requesting the client to pick from. But at which point you might as well just be using plain old un-encrypted socket connections.

EDIT: If you are just trying to send metadata along with a xhr response then you can just send custom headers along with the response and read them with getAllResponseHeaders(), just remember to set the Access-Control-Expose-Headers header too include the custom headers you want to expose to JS.

If you are trying to push data from the client to the server I guess you could do it via DNS. Make a request to http(s)://uuid.part1.thisisamessage.example.com and http(s)://uuid.part2.sentoverdns.example.com and on your dns server record the requests, resemble the parts and you have your data. You could bitbang data back to the client by having the dns reply with a server or not. It would be slow as hell (as your making a dns request and opening up a connection for each bit) but it could be done but unless you are trying to be sneaky (and it’s not that sneaky at all) it’s not worth the hassle over just opening up a “full fat” connection and reusing that connection. (Again just unable to sleep thinking out loud...)


Sure, both the ClientHello and ServerHello (which are the first two messages during setup) can contain arbitrary extension fields; these are already used for things like SNI, ALPN, "early data" (optimistic 0-RTT data), and so on. You could define an extension in the private-use range for your esoteric use.

I don't think there's any way to see this from Javscript, though. At least, not web-browser Javascript.


I'm not sure where the submitted title came from; it's not the title on the article, and it's factually incorrect -- there's only one tag (LINK), which includes an attribute (REL) that can be set five different ways. That's a lot cleaner and more orderly than "5 different tags to preload something" suggests.


We've replaced the title with that of the article. (Submitted title was "HTML has 5 different tags to preload something. Here’s a deep-dive into all".)


"TCP has 11 different state machines."


If we’re splitting hairs, I believe that according to the default SGML concrete syntax a tag is delimited by angle brackets, so both the element and the attributes are part of the tag.


Just because I can...

I find it interesting that he is inconsistent in his usage of <link>. Some of his markup has a closing slash and some of it doesn't. However, my main gripe, is that the HTML spec has never specified or required a closing slash. While it is allowed, putting that slash there has no meaning, it does nothing, and the spec tells browsers they are to ignore for those reasons.

When I ask why some people insist on putting it there, I get wildly different, unreasonable answers.

"It's just a bad habit I picked up."

How do you pick up a bad habit for something that does not exist?

"I do it so it's XHTML or XML compatible."

Well, you aren't serving it as either XHTML or XML and are very unlikely to do that. In addition, the rest of your HTML is probably not XHTML or XML compatible on top of that.

"The spec does allow it."

It allows it, as I always tell them, due to backwards compatibility with (X)HTML uses in the past, but it also says you're wasting your time and effort. See my comment at the top.

To end this, no HTML specification in the history of the universe has ever stated you need to or should put a closing slash on <link> or <img> or <input> or any other HTML tag and there is no example of such usage anywhere in said documentation either.


HTML doesn't require a closing slash, but without it needs to be context-aware and maintain a whitelist of tags that are self closing (one effect of this is that custom elements can never be self closing).

It's not really surprising that a developer would gravitate towards an encoding that is not dependent on maintaining a whitelist of tags, because that is the solution that intuitively feels more "correct".


Completely agree. If you want a generic parser for HTML-like tags, templates or anything where the context isn't a full HTML page, you definitely want syntactically correct/valid markup.


Too bad the closing slash doesn't do anything. <script src="..." /> will break horribly if you try to use it. Everything still depends on the whitelist.


If one needs a closing slash to be reminded which of the five or so tags are self closing, one has far more issues than needing a closing slash.


My point is that not that remembering things is hard, it's that it feels like a hack. Developers gravitate to solutions that feel elegant.


Putting the slash in, as I said, is there for backwards compatibility only, not elegance, and it could be removed at any time since this backwards compatibility issue no longer exists, or is of minor consequence nowadays.

It is far, far better to read and follow the standard specification and not follow a whim that makes you feel good. These are the things that will burn you first.


Issues like being a parser?


prettier-js puts them there. I don’t know why, and I don’t care, so I leave them there.


I do it because it feels right and logical. If something has a start, it should have an end. If you’re making a <div> that is only going to be used as a spacer with styling, you wouldn’t just do a <div> and leave it hanging.


You are talking about closing tags. I'm talking about self closing tags.

One should never use an element as a spacer tag. That's 1990s styling at best.


You are talking about closing tags. I'm talking about self closing tags.

I am, too. My point is that in my mind it feels right to close all tags, either with a closing tag or self-closing it with a /.

One should never use an element as a spacer tag.

One should never use “never” in programming. You are incapable of knowing every possible circumstance someone else is working in.

That's 1990s styling at best.

I built and sold several web sites in the 90’s. That’s not how it was done. It was worse.


You are putting the slash in because it feels good but the specification does not do that. You are playing with fire.

Despite your circumstance, the reason for spacer divs, and other such things, are relics of the past and such reasons no longer exist. Using them now is beyond silly.


personally, I don't care one way or the other, so I always leave them whichever was my most recently used linter/IDE's default preference & basically ignore it beyond that

the spec says to ignore it either way & I think my way aligns w/ that (while being nearly optimal in terms of effort efficiency)


Some template engines require well-formed XML (like Thymeleaf, in certain configurations)


And that is XML, not HTML, and never the twain shall meet.

The hilarity of your statement is all the disdain people had for XML and the W3C's push for its usage over HTML. Now everyone praises WHATWG for breaking away from the W3C over this and, here we are, with XML lookalikes and XML usage everywhere.




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