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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

Add External Stylesheets/Pens

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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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About Packages

Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                - var d = 2000, o = -.5*d;
- var w = .8*d, h = .1*w;
- var x = .5*(d - w), y = .5*h;
- var sw = .005*d, fs = .07*d;
- var na = Math.round(.0625*fs), pa = .2*h;

svg(viewBox=[-d, o, 2*d, d].join(' ')
		xmlns='http://www.w3.org/2000/svg' 
		xmlns:xlink='http://www.w3.org/1999/xlink')
	style
		| svg > [d], g > [d] {
		| 	fill: none;
		| 	stroke: #ccc;
		| 	stroke-width: #{sw}
		| }
		| [x] {
		| 	fill: #777;
		| 	font: #{fs}px trebuchet ms, verdana, sans-serif
		| }
		| [id], #s ~ * { fill: #333 }
		| #s ~ * { font: 700 #{fs}px courier new, monospace }
		| [d*=v] + [d] { stroke: #95a }
		| #t { stroke: none; fill: url(#p) }
		| #t + * { stroke: #b53 }
		| #t + * ~ * {
		| 	stroke: #333;
		| 	marker-start: url(#s);
		| 	marker-end: url(#e)
		| }
		| #g ~ * {
		| 	fill: #95a;
		| 	font: #{fs}px comic sans ms, noteworthy, dancing script ot, ubuntu, cursive
		| }
		| #g ~ use ~ *, #p { fill: #b53 }
	text(x=x - d y=o + x) INITIAL
	text(x=x y=o + x) FINAL
	pattern#p(width=pa height=pa patternUnits='userSpaceOnUse')
		path(d=`M${.25*pa} 0H0v${.25*pa}
						M0 ${.75*pa}v${.25*pa}h${.25*pa}l${.75*pa}-${.75*pa}V0H${.75*pa}
						M${.75*pa} ${pa}h${.25*pa}V${.75*pa}`)
	marker#s(markerWidth=na markerHeight=na 
						viewBox='-8 -4 8 8' orient='auto' refX='-7')
		path(d='M-8 0 0-4V4')
	text(x=w + x - h + .3*fs y=1.7*h) k
	text(x=h - w - x - .9*fs y=-1.3*h) k
	marker#e(markerWidth=na markerHeight=na 
					 viewBox='0 -4 8 8' orient='auto' refX='7')
		path(d='M8 0 0-4V4')
	text(x=.5*w + x - .3*fs y=3.7*h) 1
	text(x=-.2*fs - .5*w - x y=-3.2*h) 1
	g#g
		path(d=`M${x} 0v${3.5*h}
						M${w + x} 0v${3.5*h}
						M${w + x - h} 0v${2*h}`)
		path(d=`M${x}${-y}h${w}V${y}H${x}z
						M${.5*w - .25*h} ${-h}
						q${.25*h} ${.8*h} ${.5*h} 0
						M${.5*w}${-.75*h}q0${-h} ${-.5*h}${-2*h}`)
		path#t(d=`M${w + x - h}${-y}h${h}V${y}h${-h}z`)
		path(d=`M${w + x - .75*h} ${-h}
							q${.25*h} ${.8*h} ${.5*h} 0
							M${w + x - .5*h}${-.75*h}q0${-h} ${-.5*h}${-2*h}`)
		path(d=`M${w + x} ${1.5*h}h${.8*h}
						M${w + x - 1.8*h} ${1.5*h}h${.8*h}`)
		path(d=`M${x} ${3*h}h${w}`)
	text(x=.5*w - 1.5*h y=-3*h) input
	text(x=-(.5*w + .5*h) y=3*h + .8*fs) input
	use(xlink:href='#g' transform='scale(-1)')
	text(x=w - h y=-3*h) thumb
	text(x=-(w + 1.5*h) y=3*h + .8*fs) thumb

// minimal SVG
<svg viewBox='-2000 -1000 4000 2000' xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink'><style>svg>[d],g>[d]{fill:none;stroke:#ccc;stroke-width:10}[x]{fill:#777;font:140px trebuchet ms,verdana,sans-serif}[id],#s~*{fill:#333}#s~*{font:700 140px courier new,monospace}[d*=v]+[d]{stroke:#95a}#t{stroke:none;fill:url(#p)}#t+*{stroke:#b53}#t+*~*{stroke:#333;marker-start:url(#s);marker-end:url(#e)}#g~*{fill:#95a;font:140px comic sans ms,noteworthy,dancing script ot,ubuntu,cursive}#g~use~*,#p{fill:#b53}</style><text x='-1800' y='-800'>INITIAL</text><text x='200' y='-800'>FINAL</text><pattern id='p' width='32' height='32' patternUnits='userSpaceOnUse'><path d='M8 0H0v8M0 24v8h8l24-24V0H24M24 32h8V24'/></pattern><marker id='s' markerWidth='9' markerHeight='9' viewBox='-8 -4 8 8' orient='auto' refX='-7'><path d='M-8 0 0-4V4'/></marker><text x='1682' y='272'>k</text><text x='-1766' y='-208'>k</text><marker id='e' markerWidth='9' markerHeight='9' viewBox='0 -4 8 8' orient='auto' refX='7'><path d='M8 0 0-4V4'/></marker><text x='958' y='592'>1</text><text x='-1028' y='-512'>1</text><g id='g'><path d='M200 0v560M1800 0v560M1640 0v320'/><path d='M200-80h1600V80H200zM760-160q40 128 80 0M800-120q0-160-80-320'/><path id='t' d='M1640-80h160V80h-160z'/><path d='M1680-160q40 128 80 0M1720-120q0-160-80-320'/><path d='M1800 240h128M1512 240h128'/><path d='M200 480h1600'/></g><text x='560' y='-480'>input</text><text x='-880' y='592'>input</text><use xlink:href='#g' transform='scale(-1)'/><text x='1440' y='-480'>thumb</text><text x='-1840' y='592'>thumb</text></svg>
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CSS 

              
                body {
	display: flex;
	flex-direction: column;
	margin: 0;
	height: 100vh;
}
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JS
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999px

Console