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index.cp.html
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index.cp.html
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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>It's Not Dark Magic - Pulling Back the Curtains From Your Stylesheets</title>
<meta name="description" content="It's Not Dark Magic - Pulling Back the Curtains From Your Stylesheets">
<meta name="author" content="Aimee Knight">
<meta name="apple-mobile-web-app-capable" content="yes" />
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, minimal-ui">
<link rel="stylesheet" href="css/reveal.css">
<link rel="stylesheet" href="css/theme/black.css" id="theme">
<link rel="stylesheet" href="css/custom.css">
<!-- Code syntax highlighting -->
<link rel="stylesheet" href="lib/css/zenburn.css">
<!-- Printing and PDF exports -->
<script>
var link = document.createElement( 'link' );
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = window.location.search.match( /print-pdf/gi ) ? 'css/print/pdf.css' : 'css/print/paper.css';
document.getElementsByTagName( 'head' )[0].appendChild( link );
</script>
<!--[if lt IE 9]>
<script src="lib/js/html5shiv.js"></script>
<![endif]-->
</head>
<body>
<div class="reveal">
<!-- Any section element inside of this container is displayed as a slide -->
<div class="slides">
<section>
<h1>It's Not Dark Magic</h1>
<h3>Pulling Back the Curtains From Your Stylesheets</h3>
<p>
<small>Created by
<a href="http://aimeemarieknight.com">Aimee Knight</a> /
<a href="http://twitter.com/Aimee_Knight">@Aimee_Knight</a>
</small>
</p>
<aside class="notes">
</aside>
</section>
<section>
<blockquote>A language that doesn't affect the way you think about programming is not worth knowing. - Alan J. Perlis</blockquote>
<aside class="notes">
dismiss as markup lang
<br>
does affect our thinking
</aside>
</section>
<section>
<img src="img/blinds.gif" >
<aside class="notes">
Trying to fix someone else's CSS
</aside>
</section>
<section>
<img src="img/amk_skate.gif" style="width: 370px;">
<ul>
<li>Work For: Built Technologies</li>
<li>Hobbies: JavaScript Jabber, ⛸, 🏋, running</li>
<li>Likes: 😻😻, 🍵, kombucha</li>
</ul>
<aside class="notes">
NSS
</aside>
</section>
<section>
<ul>
<li>Software developers spend more time reading than writing source code</li>
<li>Knowledge of internals can be useful for advanced debugging and performance tuning</li>
<li>In the future, we'll be able to modify the browsers rendering engine ourselves!</li>
</ul>
<aside class="notes">
why it’s important
<br>
my story - front end
<br>
guessing vs systematic
</aside>
</section>
<section>
<h3>Houdini</h3>
<ul>
<li>Normalize cross-browser differences</li>
<li>Invent or polyfill new features</li>
</ul>
<aside class="notes">
APIs to extend CSS/hook into rendering engine
<br>
bootstrap: just avoids
<br>
jQuery for CSS
</aside>
</section>
<section>
<blockquote>Low-level programming is good for the programmer’s soul. – John Carmack</blockquote>
<aside class="notes">
</aside>
</section>
<section>
<h3>Can't I just use component based styling?<h3>
<aside class="notes">
</aside>
</section>
<section>
<ul>
<li>No longer able to change one variable to update all colors</li>
<li>Unable to affect children by parent classes</li>
</ul>
<aside class="notes">
gain specificty, lose the cascade
</aside>
</section>
<section>
<ul>
<li>Browser Internals</li>
<li>Parsing Process</li>
<li>Common Issues: specificity, positioning, z-index</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h3>Vocabulary</h3>
<table style="font-size: 80%">
<tr>
<td>Browsers</td>
<td>
<ul>
<li>Chromium</li>
<li>Chrome</li>
</ul>
</td>
</tr>
<tr>
<td>Rendering Engines</td>
<td>
<ul>
<li>Webkit <i>(Safari)</i></li>
<li>Blink <i>(Chrome, Opera)</i></li>
<li>Gecko <i>(Firefox)</i></li>
<li>Trident <i>(IE)</i></li>
<li>EdgeHTML <i>(Edge)</i></li>
</ul>
</td>
</tr>
<tr>
<td>JavaScript Engines</td>
<td>
<ul>
<li>Nitro <i>(Safari)</i></li>
<li>V8 <i>(Chrome)</i></li>
<li>SpiderMonkey <i>(Firefox)</i></li>
<li>Chakra <i>(IE)</i></li>
<li>Chakra <i>(Edge)</i></li>
</ul>
</td>
</tr>
</table>
<aside class="notes">
Open source browser vs rendering/layout engine
<br>
Webkit (apple) forked into blink, EdgeHTML = fork of trident
<br>
2m loc
<br>
each tab = own rendering engine
</aside>
</section>
<section>
<h1>So, how do browsers actually work? 🤔</h1>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/custom_layers.jpg">
<aside class="notes">
UI: address bar, back/fwd btn, bookmarking menu, All seen minus window.
BROWSER ENGINE: mediator.
RENDERING ENGINE: parse & display HTML CSS.
NETWORKING: HTTP requests.
UI BACKEND: drawing = OS.
JS INTERPRETER. Parse & execute JS.
DATA STORAGE: cookies, localStorage.
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/pipeline_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/dom_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/html_parser.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/how_browsers_work_breakdown.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/bytes.jpg">
<aside class="notes">
</aside>
</section>
<section>
<h1>Conversion</h1>
<ul>
<li>Reading raw bytes of HTML & CSS off of disk/network</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/characters.jpg">
<aside class="notes">
translates bytes to individual chars based on file encoding
</aside>
</section>
<section>
<img src="img/tokens.jpg">
<aside class="notes">
</aside>
</section>
<section>
<h1>Tokenizer</h1>
<ul>
<li>Breaks input into valid tokens.</li>
<li>HTML tokens: start tags, end tags, attribute names, attribute values</li>
<li>Strips irrelevant characters: white space, line breaks.</li>
</ul>
<aside class="notes">
often combined with lexing
</aside>
</section>
<section>
<h1>Tokenizer Output</h1>
<pre><code>["<", "html", ">",
"<", "head", ">",
"<", "title", ">", "My HTML Page", "</", "title", ">",
"</", "head", ">",
"<", "body", ">",
"<", "p", "style", "=", "\"", "special", "\"", ">",
"This paragraph has special style",
"</", "p", ">",
"<", "p", ">",
"This paragraph is not special",
"</", "p", ">",
"</", "body", ">",
"</", "html", ">"
]</code></pre>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/syntax_analysis.jpg">
<aside class="notes">
emitted tokens from tokenization are converted into objects
</aside>
</section>
<section>
<h1>Parser</h1>
<ul>
<li>Reads the stream of tokens.</li>
<li>Analyzes the document structure according to the language grammar.</li>
</ul>
<aside class="notes">
grammer = vocabulary + syntax rules
</aside>
</section>
<section>
<img src="img/dom.jpg">
<aside class="notes">
Compiler parses source code into a tree structure
</aside>
</section>
<section>
<h1>DOM</h1>
<ul>
<li>Not context free</li>
<li>Never get a syntax error</li>
</ul>
<aside class="notes">
when parsing, have to take into account surrounding chars, what came before or after
<br>
edge cases
</aside>
</section>
<section>
<img src="img/fall_ice.gif">
<aside class="notes">
HTML = unconventional parser
</aside>
</section>
<section>
<img src="img/jump.gif">
<aside class="notes">
CSS = conventional parser
</aside>
</section>
<section>
<h1>Recap</h1>
<ul>
<li>Browser sends HTTP request for page</li>
<li>Web server sends response</li>
<li>Browser converts response data (bytes) into tokens, via tokenization</li>
<li>Browser turns tokens into nodes</li>
<li>Browser turns nodes into the DOM tree</li>
<li>DOM tree construction finished</li>
<li>Pause for CSSOM tree construction</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/cssom_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/confused.gif" >
<aside class="notes">
DOM and CSSOM are independent data structures
<br>
</aside>
</section>
<section>
<img src="img/cssom.jpg">
<aside class="notes">
AST/tree structure
<br>
Like HTML, need to convert CSS rules into something browser can understand.
<br>
Repeat HTML process for CSS
</aside>
</section>
<section>
<h1>CSSOM</h1>
<ul>
<li>By default CSS is treated as a render blocking resource</li>
<li>Browser will hold rendering of any process until the CSSOM is constructed</li>
</ul>
<aside class="notes">
encounter link tag in head for sheet. dispatches req
<br>
STOP? scripts asking for style information during doc parsing. If style not loaded & parsed, script = wrong answers
<br>
incremental = because of cascade. RED => BLUE: jarring
</aside>
</section>
<section>
<img src="img/css_parser.jpg">
<aside class="notes">
</aside>
</section>
<section>
<h1>CSSOM</h1>
<ul>
<li>CSS files are parsed into <strong style="color: #42affa">stylesheet objects</strong></li>
<li>Each stylesheet object contains a CSS <strong style="color: #42affa">rules object</strong></li>
<li>Each rules object contains <strong style="color: #42affa">selector and declaration objects</strong> and <strong style="color: #42affa">other objects corresponding to CSS grammar</strong></li>
</ul>
<aside class="notes">
to compute final styles for obj on pg, browser starts with most general rule for a node (body element) then recursively computed styles by applying more specific rules (cascade)
</aside>
</section>
<section>
<h1>CSSOM</h1>
<ul>
<li>Not 1 to 1 with the DOM</li>
<li>Display none, script tags, meta tags, head element, etc. are omitted since they're not reflected in the rendered output</li>
</ul>
<aside class="notes">
DOM tree captures properties + relationship to doc markup
<br>
Doesn't tell how element look rendered. That’s responsibility of CSSOM
<br>
hidden visibility WILL appear
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/render_tree_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/render_tree_construction.jpg">
<aside class="notes">
tree is of visual elements in order in which displayed
</aside>
</section>
<section>
<h1>Render Tree</h1>
<h4>DOM + CSSOM</h4>
<ul>
<li>Visual representation of the document</li>
<li>Enable painting the contents in their correct order</li>
<li>CSSOM can have drastic effects on the render tree but none on the DOM tree</li>
</ul>
<aside class="notes">
since CSSOM can reorder nodes
</aside>
</section>
<section>
<h1>Purpose</h1>
<ul>
<li>Knows how to layout and paint itself and its children</li>
<li>Includes geometric information like width, height and position</li>
<li>Each node in the DOM tree that produces visual output has a corresponding render object in the render tree</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Construction</h1>
<ul>
<li>Starting at the root of the DOM tree, traverse each node</li>
<li>Omit non visible nodes</li>
<li>For each visible node find the matching CSSOM rules and apply it</li>
<li>Emit visible nodes with content and computed styles</li>
<li>Output a render tree that contains both the content and style information of all visible content on the screen</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Rendering Phases</h1>
<ul>
<li>Layout ↓</li>
<li>Paint ↓</li>
<li>Composite ↓</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/layout_diagram.jpg">
<aside class="notes">
Chrome DevTools perf tab capture
</aside>
</section>
<section>
<img src="img/dev_tools_layout.png" style="width: 80%">
<aside class="notes">
</aside>
</section>
<section>
<h1>Layout</h1>
<h4>(AKA Reflow)</h4>
<ul>
<li>Calculating the position and size of the tree's render objects</li>
<li>The output of the layout process is a box model that contains the position and size of every element within the viewport</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/paint_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/dev_tools_paint.png" style="width: 80%">
<aside class="notes">
</aside>
</section>
<section>
<h1>Paint</h1>
<h4>(AKA Rasterizing)</h4>
<ul>
<li>Converts each node in the render tree to actual pixels on the screen</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Painting</h1>
<ul>
<li>Multiple rounds of painting can be caused by JS being loaded that changes the DOM</li>
<li>Includes: text, colors, shadows, background images, etc.</li>
<li>Changing any property besides transform or opacity will always trigger paint</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Painting Time</h1>
<ul>
<li>Time varies based on render tree construction</li>
<li>The bigger the width and height of the element, the longer the painting time</li>
<li>Adding different effects also increases painting time</li>
<li>Paint is usually the most expensive component of the render pipeline</li>
</ul>
<aside class="notes">
(2) first thing to look for is the area of the element
</aside>
</section>
<section>
<h1>Painting Time</h1>
<ul>
<li>Float: 14ms</li>
<li>Flexbox: 3.5ms</li>
</ul>
<aside class="notes">
<br>
flexbox is great from a usability standpoint
<br>
experiment
<br>
keep up with css skills
</aside>
</section>
<section>
<h1>Painting Order</h1>
<ul>
<li>Follows order that elements are stacked in their stacking contexts</li>
<li>Painted from back to front</li>
<li>Can be global or incremental</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/repaint.gif">
<aside class="notes">
Incremental: when styles change render obj becomes invalid = dirty region = generate paint
</aside>
</section>
<section>
<h3>Rendering Engine Main Flow</h3>
<img src="img/composite_diagram.jpg">
<aside class="notes">
</aside>
</section>
<section>
<img src="img/composite_dev_tools.png">
<aside class="notes">
</aside>
</section>
<section>
<h1>Composite</h1>
<ul>
<li>Render different elements of a web page in different layers (think photoshop layers)</li>
<li>Action of flattening all layers into the final image that is visible on the screen</li>
</ul>
<aside class="notes">
hardware acceleration = compositing
<br>
GPU = speed up vs old CPU
<br>
cache & group chunks of render tree
<br>
painted parts put together
</aside>
</section>
<section>
<h3>Rendering performance</h3>
<ul>
<li>Changes to the element's position will cause relayout and repaint of that element, its children and possibly siblings</li>
<li>Adding a DOM node will cause relayout and repaint of the parent node</li>
<li>Major changes, like increasing font size of the html element will cause relayout and repaint of the entire tree</li>
<li>Changes to an element's color will only repaint that element</li>
<li>Transforms and opacity are currently the only properties which only trigger composite</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Summary</h1>
<ul>
<li>Browsers try to do the minimal possible actions in response to a change</li>
<li>Layout always triggers paint since changing the geometry of element means the pixels need to be fixed</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/delete.gif" >
<aside class="notes">
ever moved something around only to see another part of page break?
</aside>
</section>
<section>
<img src="img/tweet.png" >
<aside class="notes">
me!
</aside>
</section>
<section>
<blockquote>The amateur software engineer is always in search of magic. - Grady Booch</blockquote>
<aside class="notes">
</aside>
</section>
<section>
<h1>Specificity</h1>
<ul>
<li>Many ways to target a specific tag using CSS selectors</li>
<li>Applying rules in correct cascade order</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Specificity</h1>
<ul>
<li>If multiple CSS selectors are targeting the same set of HTML elements, and if the CSS selectors are trying to assign the same property/properties to the HTML elements, the selector with the highest specificity value will win</li>
<li>Browser needs a way to negotiate which style it should give to a specific tag</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Specificity</h1>
<ul>
<li>Browser makes a decision by first calculating each selectors' specificity value</li>
<li>Second, it checks which selector has the highest value to determine the winner</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h3>Selector Prerequisite Refresher</h3>
<table>
<tr>
<th>
<h4>Type</h4>
</th>
<th>
<h4>Examples</h4>
</th>
</tr>
<tr>
<td>Attribute selectors</td>
<td>
<ul>
<li>[target="blank"]</li>
<li>[type="text"]</li>
</ul>
</td>
</tr>
<tr>
<td>Pseudo-classes</td>
<td>
<ul>
<li>:hover</li>
<li>:visited</li>
</ul>
</td>
</tr>
<tr>
<td>Pseudo-elements</td>
<td>
<ul>
<li>::before</li>
<li>::after</li>
</ul>
</td>
</tr>
</table>
<aside class="notes">
all know type (div)
</aside>
</section>
<section>
<pre><code><div class="container">
<div id="main">
<p>
<a href="#">Link</a>
</p>
</div>
</div></code></pre>
<pre><code>#main a {
color: green;
}
p a {
color: yellow;
}
.container #main a {
color: pink;
}
div #main p a {
color: orange;
}
a {
color: red;
}</code></pre>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/sponge_bob.gif">
<h1 class="fragment fade-in">Answer: <span style="color:#FF69B4;">Pink!</span></h1>
</section>
<section>
<h1>Specificity</h1>
<ul>
<li>You can read values as if they were a number. 0,1,1,1 = 111.</li>
<li>Commas are there to remind us that it's not a base 10 system.</li>
<li>Can have a specificity value of 0,1,13,4 and 13 won't spill over like a base 10 system.</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<table style="font-size: 70%">
<tr>
<th>
<h5>Selector</h5>
</th>
<th>
<h5>Specificity Value (<span style="color: #9932CC">A</span>,<span style="color: #00BFFF">B</span>,<span style="color: #7CFC00">C</span>,<span style="color: #EE82EE">D</span>)</h5>
</th>
</tr>
<tr>
<td>.container #main a</td>
<td><span style="color: #EE82EE">0</span>,<span style="color: #9932CC">1</span>,<span style="color: #00BFFF">1</span>,<span style="color: #7CFC00">1</span></td>
</tr>
<tr>
<td>div #main p a</td>
<td><span style="color: #EE82EE">0</span>,<span style="color: #9932CC">1</span>,<span style="color: #00BFFF">0</span>,<span style="color: #7CFC00">3</span></td>
</tr>
<tr>
<td>#main a</td>
<td><span style="color: #EE82EE">0</span>,<span style="color: #9932CC">1</span>,<span style="color: #00BFFF">0</span>,<span style="color: #7CFC00">1</span></td>
</tr>
<tr>
<td>p a</td>
<td><span style="color: #EE82EE">0</span>,<span style="color: #9932CC">0</span>,<span style="color: #00BFFF">0</span>,<span style="color: #7CFC00">2</span></td>
</tr>
<tr>
<td>a</td>
<td><span style="color: #EE82EE">0</span>,<span style="color: #9932CC">0</span>,<span style="color: #00BFFF">0</span>,<span style="color: #7CFC00">1</span></td>
</tr>
</table>
<p class="fragment fade-in" style="color: #EE82EE; font-size: 40px !important">A: Styles attribute (1,0,0,0 points)</p>
<p class="fragment fade-in" style="color: #9932CC; font-size: 40px !important">B: Number of ID selectors (0,1,0,0 points)</p>
<p class="fragment fade-in" style="color: #00BFFF; font-size: 40px !important">C: Number of class selectors, attribute selectors, pseudo-classes (0,0,1,0 points)</p>
<p class="fragment fade-in" style="color: #7CFC00; font-size: 40px !important">D: Number of type selectors, pseudo-elements (0,0,0,1 points)</p>
<aside class="notes">
had to write down my calculations on a piece of paper
</aside>
</section>
<section>
<h4>Specificity value: 0,1,2,2</h4>
<pre><code>#header .navbar li a:visited</code></pre>
<table>
<tr>
<th>A</th>
<th>B</th>
<th>C</th>
<th>D</th>
</tr>
<tr>
<td>0</td>
<td>1</td>
<td>2</td>
<td>2</td>
</tr>
</table>
<aside class="notes">
0 style attrs, 1 id, 1 class + 1 pseudo-class, 2 type selectors (li, a)
</aside>
</section>
<section>
<h1>Positioning</h1>
<img src="img/css_is_awesome.jpg">
<aside class="notes">
process of determining the position and size of all boxes = going back to layout
</aside>
</section>
<section>
<h1>Positioning</h1>
<h4>Normal & Relative</h4>
<ul>
<li>Normal: The object is positioned according to its place in the document, box type, and dimensions</li>
<li>Relative: Positioned like usual and then moved by the required delta</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Positioning</h1>
<h4>Float</h4>
<ul>
<li>The object is first laid out in normal flow, then moved as far left or right as possible</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Positioning</h1>
<h4>Absolute</h4>
<ul>
<li>The object is put in the render tree in a different place than in the DOM tree</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Positioning Phases</h1>
<ul>
<li>Layout is a recursive process</li>
<li>Width: Computed by the current render object</li>
<li>Height: For each child, the current render object determines the position of the child, asks the child to compute its dimensions, then the current render object can compute its height</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<img src="img/layout.jpg" style="width: 80%">
<aside class="notes">
parent tells child to fit into it’s width
<br>
1. Width, 2. Height
</aside>
</section>
<section>
<h1>Global Layout</h1>
<ul>
<li>Layout can be triggered on the entire render tree as a result of a global style change that affects all render objects</li>
<li>Triggered synchronously</li>
<li>Example: font size change, screen being resized</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Incremental layout</h1>
<ul>
<li>Only dirty render objects will be laid out</li>
<li>Triggered asynchronously</li>
<li>Example: new render objects are appended to the render tree after extra content came from the network and was added to the DOM tree</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Z -Index</h1>
<p>Every element in an HTML document can be either in front of or behind every other element in the document</p>
<aside class="notes">
elements with higher z-index are stacked in front of elements with lower z-index
</aside>
</section>
<section>
<img src="img/so_hard.gif" >
<aside class="notes">
appears so simple, so most devs don’t dig deeper
</aside>
</section>
<section>
<h1>Stacking Context</h1>
<ul>
<li>A three-dimensional representation of HTML elements along an imaginary z-axis relative to the user facing the viewport</li>
<li>An element that contains a set of layers</li>
<li>Groups of elements with a common parent that move forward or backward together</li>
</ul>
<aside class="notes">
understanding of stacking contexts is key
</aside>
</section>
<section>
<h1>Stacking Order</h1>
<ul>
<li>Every stacking context has a single HTML element as its root element</li>
<li>When z-index and position properties aren’t involved, the rules are simple: the stacking order is the same as the order of appearance in the HTML</li>
</ul>
<aside class="notes">
POSITIONED: relative, absolute, etc. (value other than static)
</aside>
</section>
<section>
<h1>Stacking Context</h1>
<ul>
<li>Root Stacking Context: created by an html element</li>
<li>Local Stacking Context: created by specific properties and values</li>
<li>Within a local stacking context, the z-index values of children are relative to the root element rather than to the document root</li>
<li>Layers outside of a context (sibling elements of a local stacking context) can't sit between layers within another context</li>
</ul>
<aside class="notes">
</aside>
</section>
<section>
<h1>Stacking Context</h1>
<ul>
<li>When a new stacking context is formed on an element, that stacking context confines all of its child elements to a particular place in the stacking order</li>
<li>If an element is contained in a stacking context at the bottom of the stacking order, there is no way to get it to appear in front of another element in a different stacking context</li>
</ul>
<aside class="notes">
<br>
diff contexts ? can't even change even with a z-index of billion!