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ericlaw talks about the web and software in general

2020-02-09

Demystifying Browsers

I started building browser extensions more than 22 years ago, and I
started building browsers directly just over 16 years ago. At this
point, I think it’s fair to say that I’m entering the grizzled veteran
phase of my career.

With the Edge team [11]continuing to grow with bright young minds from
college and industry, I’m increasingly often asked “Where do I learn
about browsers?” and I haven’t had a ready answer for that question.

This post aims to answer it.

First, a few prerequisites for developing expertise in browsers:
1. Curiosity. While browsers are more complicated than ever, there are
also better resources than ever to learn how they work. All major
browsers are now based on open-source code, and if you’re curious,
you no longer need to [12]join a secret priesthood to discover how
they operate under the hood.
2. Willingness to Experiment. Considering how complex browsers are
(and because they’re so diverse, across platforms, maker, and
version), it’s often easiest to definitively answer questions about
how browsers work by trying things, rather than reading an
explainer (possibly outdated or a map that doesn’t match the
terrain) or reading the code (often complex and potentially
misleading). Build test cases and try them in each browser to see
what happens. When you encounter [13]surprising [14]behavior, let
your curiosity guide you into figuring it out. Browsers contain no
magic, but plenty of [15]butterfly effects.
3. Doggedness. I’ve been doing this for half of my life, and I’m still
learning daily. While historical knowledge will serve you well,
things are changing in this space every day, and keeping up is an
endless challenge. And it’s often fun.

Now, how do you apply these prerequisites and grow to become a master
of browsers? Read on.

Fundamental Understanding

Over the years, a variety of broad resources have been developed that
will give you a good foundation in the fundamentals of how browsers
work. Taking advantage of these will help you more effectively explore
and learn on your own.
* First, I recommend reading [16]the Chrome Comic Book. This short,
38 page comic book from legend Scott McCloud was published
alongside the first version of Google Chrome back in 2008. It
clearly and simply explains many of the core concepts behind modern
browsers as application platforms.
* HTML5Rocks has a great introduction into [17]How Browsers Work.
This is a lengthy and detailed introduction into how browsers turn
HTML and CSS into what you see on the screen. Read this article and
you’ll understand more about this topic than 90% of web developers.
* The folks at Google have created a fantastic four-part illustrated
series about how modern browsers work: [18]Inside look at modern
web browsers. [19]Navigation, [20]the Rendering Engine and
[21]Input and Compositing as a part of their [22]Web Fundamentals
site.
* Mozilla wrote a fantastic [23]cartoon introduction to WebAssembly,
explaining the basics behind this new technology; there’s tons of
other invaluable content on [24]Mozilla Hacks.
* [25]The Chromium Chronicle is a monthly series geared specifically
to the Chromium developers who build the browser.
* The [26]Chrome University [27]2[28]0[29]1[30]9 (and [31]2018) talks
on YouTube are amazing.
* Web Developers should check out [32]Web.Dev, a great source of
articles on building fast and secure websites.

Books

If you prefer to learn from books, I can recommend a few. Sadly, there
are few on browsers themselves (largely because they tend to evolve too
quickly), but there are good books on web technologies.
* [33]High Performance Browser Networking can be [34]read online for
free.
* [35]HTTP2 in Action explains this important new protocol.
* [36]The Tangled Web explains the web security model.
* [37]Progressive Web Apps explores the technologies behind this new
web application paradigm.

Tools

One of the best ways to examine what’s going on with browsers is to
just use tools to watch what’s going on as you use your favorite
websites.
* Built-in DevTools (just hit F12!) – They’re amazingly powerful. I
don’t know of a great tutorial, but there are likely some [38]on
YouTube.
* [39]Telerik Fiddler – See what requests hit the network and what
they contain.
* [40]The VisBug Chrome Extension – Easily manipulate any page
layout, directly in your browser.

Use the Source, Leia

The fact that all of the major browsers are built atop open-source
projects is a wonderful thing. No longer do you need to be a
reverse-engineering ninja with a low-level debugger to figure out how
things are meant to work (although sometimes such approaches can still
be super-valuable).

Source code locations:
* Firefox‘s code can be searched via [41]Searchfox or [42]DXR.
* Webkit source can be found [43]here, although I typically just
search on a [44]mirror on GitHub.
* Chromium‘s code is found in the [45]Code Search tool ([46]future
version).
* Microsoft Edge‘s code can be downloaded in giant (4gb) ZIP files
from [47]here. Sadly, online search is not yet available.
* [48]Brave‘s changes to Chromium can be found on GitHub.
* The [49]HTML5 Specification is often useful.

Navigating the Code

While simply perusing a browser’s source code might give you a good
feel for the project, browsers tend to be enormous. Chromium is over 10
million lines of code, for example.

If you need to find something in particular, one often effective way to
find it easily is to search for a string shown in the browser UI near
the feature of interest. (Or, if you’re searching for a DOM function
name or HTML attribute name, try searching for that.)

By way of example, today I encountered an unexpected behavior in the
handling of the “Go to <url>” command on Chromium’s context menu:

So, to find the code that implements this feature, I first try
searching for that string:

…but there are a gazillion hits, which makes it hard to find what I
need. So I instead search for a string that’s elsewhere in the context
menu, and find only one hit in the Chromium “grd” (resources) file:

When I go look at that grd file, I quickly find the identifier I’m
really looking for just below my search result:

So, we now know that we’re looking for usages of
IDS_CONTENT_CONTEXT_GOTOURL, probably in a .CC file, and [50]we find
that almost immediately:

From here, we see that the menu item has the command identifier
IDC_CONTENT_CONTEXT_GOTOURL, which we can then continue to chase down
through the source until we find the code [51]that handles the command.
That command makes use of a variable selection_navigation_url_, which
is [52]filled elsewhere by some pretty complicated logic.

After you gain experience in the Chromium code, you might learn “Oh,
yeah, all of the context menu stuff is easy to find, it’s in the
renderer_context_menu directory” and limit your searches to that area,
but after four years of working on Chrome, I still usually start my
searches broadly.

Look at their Bugs

All browsers except Microsoft Edge have a public bug tracker where you
can search for known issues and file new bugs if you encounter them.
* Firefox – [53]Firefox Bugzilla
* Webkit – [54]WebKit Bugzilla
* Chromium – [55]CRBug
* Microsoft Edge‘s – Platform bugs that are inherited from Chromium
are tracked using [56]CRBug. Sadly, at present there is no public
tracker for bugs that reproduce only in Edge. Bugs reported by the
“Feedback” button are tracked internally by Microsoft.
* Brave – [57]on GitHub
* HTML5 Specification – [58]on GitHub

Blogs to Read

* [59]This One – I write mostly about browsers.
* My (archived) [60]IEInternals – I started writing this blog because
it was the only reliable way for me to find my notes from
investigations and troubleshooting years later.
* [61]Cloudflare – Cloudflare is a $5B company whose primary product
is their amazing blog. I understand they also run a CDN on the side
to generate interesting topics for their blog to talk about.
* [62]Nasko Oskov’s – Nasko is an engineer on the Chrome Security
team and writes mostly about security topics.
* [63]Chris Palmer’s – Chris is an engineer on the Chrome Security
team and writes about secure design.
* [64]Adam Langley’s – Google’s expert cryptographer
* [65]Bruce Dawson’s – Bruce is a Chrome Engineer who posts lots of
interesting information about debugging and performance
troubleshooting, especially on Windows.
* [66]Anne van Kesteren’s – Anne works on the HTML5 spec.
* [67]Mark Nottingham’s – Mark co-chairs the HTTP and QUIC working
groups

People to Follow

* Lin Clark ([68]@linclark) draws [69]@codecartoons. These are my
favorite thing in the universe.
* Similarly, [70]@kosamari provides some of the clearest explanations
of complicated browser topics.
* The [71]@intenttoship bot publishes notices of browsers’ intention
to launch new features
* Other Chrome folks often tweet interesting things: [72]Mike West,
[73]Alex Russell, [74]Paul Irish, [75]Jake Archibald, [76]Rick
Byers, [77]Pete LePage, [78]Surma, [79]Jochen Eisinger, [80]Jeffrey
Yasskin, [81]Yoav Weiss, [82]Chris Wilson, [83]Domenic Denicola,
[84]Paul Kinlan, [85]Adrienne Porter Felt, [86]Emily Schechter,
[87]Emily Stark, [88]Parisa Tabriz, [89]Nasko Oskov, [90]Simeon,
[91]Ryan Sleevi, [92]Justin Schuh, [93]Rowan Merewood, [94]Peter
Beverloo, [95]Dominick Ng, [96]Asanka Herath, [97]Darin Fisher,
[98]Andrew Whalley, [99]Dion Almaer, [100]Mathias Bynens,
[101]David Benjamin, [102]Bruce Dawson, [103]Addy Osmani, [104]Ilya
Grigorik, [105]Mathieu Perreault, [106]Varun Khaneja, [107]Chris
Thompson, [108]Ian Kilpatrick, [109]Avi Drissman
* [110]PPK, [111]Michal Špaček, [112]François Remy, [113]Melanie
Richards, [114]Tony Ross, [115]Mark Nottingham, [116]Alex Wykoff,
[117]Mike Taylor, [118]Adam Langley, [119]Nic Jansma, [120]John
Wilander, [121]othermaciej, [122]pes, [123]Artur Janc, [124]Ivan
Fratric, [125]Lucas Pardoe, [126]Anne van Kesteren, [127]Aaron
Gustafson, [128]Allen Wirfs-Brock, [129]J.C., [130]Adam Roach,
[131]Zouhir, [132]Kamila Hasanbega, [133]Matt Holt, [134]John
Graham-Cumming, [135]Jun Kokatsu, [136]Patrick Kettner, [137]Eiji
Kitamura, [138]Jonathan Kingston, [139]Sampson, [140]Brian Clifton,
[141]Brian Bondy, [142]Owen Campbell-Moore, [143]Natalie
Silvanovich, [144]Alex Ainslie, [145]Mike Conley, [146]Tanvi,
[147]Malte Ubl, [148]Chris Heilmann, [149]Monica Dinculescu,
[150]Eric Mill, [151]Evan J, [152]eae, [153]Scott Helme, [154]April
King, [155]Matthew Prince, [156]Yan, [157]Patrick Donahue,
[158]Justin, [159]Jyrki Alakuijala, [160]James Forshaw, [161]Tim
Kadlec, [162]Kyle Pflug, [163]Sam Sneddon, [164]David Storey,
[165]Chris Love, [166]Travis Leithead, [167]Tammy Everts, [168]Greg
Whitworth, [169]Sean Lyndersay, [170]Daniel Stenberg, [171]L. David
Baron, [172]TJ VanToll, [173]John Jansen, [174]Patrick McManus,
[175]Giorgio Maone, [176]Ben Adida, [177]Ivan Ristic, [178]Patrick
Meenan, [179]Steve Souders, [180]Jungkee Song, [181]Dane Knecht,
[182]Scott Low
* [183]EdgeDevTools, [184]ChromeDevTools, [185]WebKit, [186]WebCompat
* [187]Me :)

I’ve doubtless forgotten some, [188]see who I follow.

Extra Resources

* [189]CanIUse.com often has useful information.
* [190]ChromeStatus and particularly their “[191]upcoming changes”
schedule.
* Chrome’s [192]OmahaProxy and [193]ChromiumDash.
* [194]Chromium Design Docs – Some are badly outdated, but all of
them are interesting.
* [195]SSLLabs.com

Browsers are hugely complicated beasts, and tons of fun. If the
resources above leave you feeling both overwhelmed and excited, maybe
you should become a browser builder.

Want to change the world? [196]Come join the new Microsoft Edge team
today!

-Eric
PS: Have I forgotten or overlooked a great resource for folks entering
our field? Please leave me a comment!

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About ericlaw

Impatient optimist. Dad. Author/speaker. Created Fiddler & SlickRun. PM
@ MSFT '01-'12, and '18-, presently working on Microsoft Edge. My words
are my own.

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224. https://textslashplain.com/2020/02/09/demystifying-browsers/#comment-form-load-service:Twitter
225. https://textslashplain.com/2020/02/09/demystifying-browsers/#comment-form-load-service:Facebook