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<div id="content">
<h1 class="title">Gradient Blend</h1>
<div id="outline-container-org4718d59" class="outline-2">
<h2 id="org4718d59"><span class="section-number-2">1</span> Gradient Blend</h2>
<div class="outline-text-2" id="text-1">
</div>
<div id="outline-container-orga82fe77" class="outline-3">
<h3 id="orga82fe77"><span class="section-number-3">1.1</span> The Problem</h3>
<div class="outline-text-3" id="text-1-1">
</div>
<div id="outline-container-orgc3e8c71" class="outline-4">
<h4 id="orgc3e8c71"><span class="section-number-4">1.1.1</span> Challenge 1</h4>
<div class="outline-text-4" id="text-1-1-1">
<p>
Write a function that given a DOM element makes it’s background-color start fading into different colors along the visible light spectrum in an endless cycle. Have the cycle length be configurable
</p>
<div class="org-src-container">
<pre class="src src-typescript"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">startGradientFade</span> = <span style="color: #AE81FF;">(</span>element: <span style="color: #66D9EF;">DOMElement</span>, cycleLengthMilliseconds: <span style="color: #66D9EF;">Number</span><span style="color: #AE81FF;">)</span>: <span style="color: #F92672;">void</span> <span style="color: #F92672;">=></span> <span style="color: #AE81FF;">{</span> ... <span style="color: #AE81FF;">}</span>
</pre>
</div>
<p>
If <code>cycleLengthMilliseconds</code> is passed a cycle length of <code>5000</code>, this will traverse the gradient from red to purple in and back to red in 5 seconds.
</p>
<img alt="The visible light spectrum" src="https://www.thoughtco.com/thmb/qP1_h_MKsrmAlx_MK-hDOasJXPY=/768x0/filters:no_upscale():max_bytes(150000):strip_icc():format(webp)/the-visible-light-spectrum-2699036_FINAL2-c0b0ee6f82764efdb62a1af9b9525050.png" />
</div>
</div>
<div id="outline-container-org14fd9da" class="outline-4">
<h4 id="org14fd9da"><span class="section-number-4">1.1.2</span> Challenge 2</h4>
<div class="outline-text-4" id="text-1-1-2">
<p>
Write a generator function that takes a “step” size (0 <= number <= 1) and produces an endless sequence of RGB color strings (eg `a562fe`) moving along the visible light spectrum where the `step` determines how large a step along the spectrum you are taking:
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-right" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-right">Step</th>
<th scope="col" class="org-left">Sequence</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-right">1</td>
<td class="org-left">ff0000, ff0000, ….</td>
</tr>
<tr>
<td class="org-right">.5</td>
<td class="org-left">ff0000, 00ffff, ff0000, …</td>
</tr>
</tbody>
</table>
<p>
(a reasonable step size to get a nice gradient might be something like `0.01` or even smaller)
</p>
<div class="org-src-container">
<pre class="src src-typescript"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">gradientColors</span> = <span style="color: #F92672;">function</span> * <span style="color: #AE81FF;">(</span>stepSize: <span style="color: #66D9EF;">Number</span><span style="color: #AE81FF;">)</span> : <span style="color: #66D9EF;">Iterable</span><<span style="color: #F92672;">string</span>> <span style="color: #AE81FF;">{</span> ... <span style="color: #AE81FF;">}</span>
</pre>
</div>
<p>
Use this function to implement the method from <a href="#orgc3e8c71">1.1.1</a>.
</p>
</div>
</div>
</div>
<div id="outline-container-org00aa750" class="outline-3">
<h3 id="org00aa750"><span class="section-number-3">1.2</span> Solution</h3>
<div class="outline-text-3" id="text-1-2">
<p>
An important thing here is to know about <code>hsl</code> / <code>hsv</code> notation.
</p>
<p width="500">
<img src="./hsl-color-wheel.png" alt="hsl-color-wheel.png" width="500" /><sup><a id="fnr.1" class="footref" href="#fn.1">1</a></sup>
</p>
<p>
In this notation, the color space is pictured as a cylinder with hue (degrees around the center) basically corresponding to the color spectrum.
</p>
<p>
<a href="https://developer.mozilla.org/en-US/docs/Web/CSS/color_value">Oh look, there's a css function for it</a>
</p>
</div>
<div id="outline-container-org52f3e7b" class="outline-4">
<h4 id="org52f3e7b"><span class="section-number-4">1.2.1</span> Challenge 1</h4>
<div class="outline-text-4" id="text-1-2-1">
<p>
Well, this is solvable with css is it not?
</p>
<article id="gradient-colors">
<div></div>
<style>
#gradient-colors div {
width: 400px;
height: 200px;
animation: gradient 5s infinite linear;
}
@keyframes gradient {
0% {
background-color: hsl(0, 100%, 50%);
}
25% {
background-color: hsl(90, 100%, 50%);
}
50% {
background-color: hsl(180, 100%, 50%);
}
75% {
background-color: hsl(270, 100%, 50%);
}
100% {
background-color: hsl(360, 100%, 50%);
}
}
</style>
</article>
<div class="org-src-container">
<pre class="src src-html"><<span style="color: #A6E22E;">article</span> <span style="color: #FD971F;">id</span>=<span style="color: #E6DB74;">"gradient-colors"</span>>
<<span style="color: #A6E22E;">div</span>></<span style="color: #A6E22E;">div</span>>
<<span style="color: #A6E22E;">style</span>>
#gradient-colors div {
width: 400px;
height: 200px;
animation: gradient 5s infinite linear;
}
@keyframes gradient {
0% {
background-color: hsl(0, 100%, 50%);
}
25% {
background-color: hsl(90, 100%, 50%);
}
50% {
background-color: hsl(180, 100%, 50%);
}
75% {
background-color: hsl(270, 100%, 50%);
}
100% {
background-color: hsl(360, 100%, 50%);
}
}
</<span style="color: #A6E22E;">style</span>>
</<span style="color: #A6E22E;">article</span>>
</pre>
</div>
<p>
That works! To put it behind a function I would simply have the function emit a similar stylesheet. Lets just move on to…
</p>
</div>
</div>
<div id="outline-container-org420fc2d" class="outline-4">
<h4 id="org420fc2d"><span class="section-number-4">1.2.2</span> Challenge 2</h4>
<div class="outline-text-4" id="text-1-2-2">
<p>
To be honest this is not much harder than the implementation of challenge 1
</p>
<div class="org-src-container">
<pre class="src src-js"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">gradientColors</span> = <span style="color: #F92672;">function</span> * <span style="color: #AE81FF;">(</span>stepSize<span style="color: #AE81FF;">)</span> <span style="color: #AE81FF;">{</span>
<span style="color: #F92672;">let</span> <span style="color: #FD971F;">nextHueProportion</span> = <span style="color: #AE81FF;">0</span>
<span style="color: #F92672;">while</span><span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">true</span><span style="color: #66D9EF;">)</span> <span style="color: #66D9EF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">hue</span> = Math.floor<span style="color: #A6E22E;">(</span><span style="color: #AE81FF;">360</span> * nextHueProportion<span style="color: #A6E22E;">)</span>
<span style="color: #F92672;">yield</span> <span style="color: #E6DB74;">`hsl(${hue}, 100%, 50%)`</span>
nextHueProportion += stepSize
nextHueProportion -= Math.floor<span style="color: #A6E22E;">(</span>nextHueProportion<span style="color: #A6E22E;">)</span>
<span style="color: #66D9EF;">}</span>
<span style="color: #AE81FF;">}</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">it</span> = gradientColors<span style="color: #AE81FF;">(</span>.<span style="color: #AE81FF;">05</span><span style="color: #AE81FF;">)[</span>Symbol.iterator<span style="color: #AE81FF;">]()</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">firstTen</span> = Array.from<span style="color: #AE81FF;">(</span><span style="color: #66D9EF;">{</span>length: <span style="color: #AE81FF;">10</span><span style="color: #66D9EF;">}</span>, <span style="color: #66D9EF;">()</span> => it.next<span style="color: #66D9EF;">()</span>.value<span style="color: #AE81FF;">)</span>
console.log<span style="color: #AE81FF;">(</span>firstTen<span style="color: #AE81FF;">)</span>
</pre>
</div>
<pre class="example">
[ 'hsl(0, 100%, 50%)',
'hsl(18, 100%, 50%)',
'hsl(36, 100%, 50%)',
'hsl(54, 100%, 50%)',
'hsl(72, 100%, 50%)',
'hsl(90, 100%, 50%)',
'hsl(108, 100%, 50%)',
'hsl(125, 100%, 50%)',
'hsl(144, 100%, 50%)',
'hsl(161, 100%, 50%)' ]
</pre>
<p>
Due to how the DOM/CSS interact, all of the above are perfectly valid colors to use.
</p>
<p>
However where's the fun in that? We actually want RGB format. <a href="https://www.rapidtables.com/convert/color/hsl-to-rgb.html">This is not entirely straightforward</a> but has some reasonable equations.
</p>
<div class="org-src-container">
<pre class="src src-js" id="orga18f952"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">isInOrder</span> = <span style="color: #AE81FF;">(</span>min, v, max<span style="color: #AE81FF;">)</span> => min <= v && v < max
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">toHex</span> = <span style="color: #AE81FF;">(</span>num<span style="color: #AE81FF;">)</span> => Math.floor<span style="color: #AE81FF;">(</span>num<span style="color: #AE81FF;">)</span>.toString<span style="color: #AE81FF;">(</span><span style="color: #AE81FF;">16</span><span style="color: #AE81FF;">)</span>.toUpperCase<span style="color: #AE81FF;">()</span>.padStart<span style="color: #AE81FF;">(</span><span style="color: #AE81FF;">2</span>, <span style="color: #E6DB74;">`0`</span><span style="color: #AE81FF;">)</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">hslToRgb</span> = <span style="color: #AE81FF;">(</span>h, s, l<span style="color: #AE81FF;">)</span> => <span style="color: #AE81FF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">c</span> = <span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">1</span> - Math.abs<span style="color: #A6E22E;">(</span><span style="color: #AE81FF;">2</span>*l - <span style="color: #AE81FF;">1</span><span style="color: #A6E22E;">)</span><span style="color: #66D9EF;">)</span> * s
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">x</span> = c * <span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">1</span> - Math.abs<span style="color: #A6E22E;">(</span> <span style="color: #E6DB74;">(</span><span style="color: #FD971F;">(</span>h/<span style="color: #AE81FF;">60</span><span style="color: #FD971F;">)</span> % <span style="color: #AE81FF;">2</span><span style="color: #E6DB74;">)</span> - <span style="color: #AE81FF;">1</span><span style="color: #A6E22E;">)</span><span style="color: #66D9EF;">)</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">m</span> = l - c/<span style="color: #AE81FF;">2</span>
<span style="color: #F92672;">const</span> <span style="color: #66D9EF;">[</span><span style="color: #FD971F;">rp</span>, <span style="color: #FD971F;">gp</span>, <span style="color: #FD971F;">bp</span><span style="color: #66D9EF;">]</span> =
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">0</span>, h, <span style="color: #AE81FF;">60</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span>c, x, <span style="color: #AE81FF;">0</span><span style="color: #66D9EF;">]</span> :
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">60</span>, h, <span style="color: #AE81FF;">120</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span>x, c, <span style="color: #AE81FF;">0</span><span style="color: #66D9EF;">]</span> :
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">120</span>, h, <span style="color: #AE81FF;">180</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span><span style="color: #AE81FF;">0</span>, c, x<span style="color: #66D9EF;">]</span> :
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">180</span>, h, <span style="color: #AE81FF;">240</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span><span style="color: #AE81FF;">0</span>, x, c<span style="color: #66D9EF;">]</span> :
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">240</span>, h, <span style="color: #AE81FF;">300</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span>x, <span style="color: #AE81FF;">0</span>, c<span style="color: #66D9EF;">]</span> :
isInOrder<span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">300</span>, h, <span style="color: #AE81FF;">360</span><span style="color: #66D9EF;">)</span> ? <span style="color: #66D9EF;">[</span>c, <span style="color: #AE81FF;">0</span>, x<span style="color: #66D9EF;">]</span> :
<span style="color: #66D9EF;">(</span><span style="color: #A6E22E;">()</span> => <span style="color: #A6E22E;">{</span> <span style="color: #F92672;">throw</span> Error<span style="color: #E6DB74;">(</span><span style="color: #E6DB74;">`Hue must be 0 <= #{h} < 360`</span><span style="color: #E6DB74;">)</span><span style="color: #A6E22E;">}</span><span style="color: #66D9EF;">)()</span>
<span style="color: #F92672;">const</span> <span style="color: #66D9EF;">[</span><span style="color: #FD971F;">r</span>, <span style="color: #FD971F;">g</span>, <span style="color: #FD971F;">b</span><span style="color: #66D9EF;">]</span> = <span style="color: #66D9EF;">[</span><span style="color: #A6E22E;">(</span>rp+m<span style="color: #A6E22E;">)</span>*<span style="color: #AE81FF;">255</span>, <span style="color: #A6E22E;">(</span>gp+m<span style="color: #A6E22E;">)</span>*<span style="color: #AE81FF;">255</span>, <span style="color: #A6E22E;">(</span>bp+m<span style="color: #A6E22E;">)</span>*<span style="color: #AE81FF;">255</span><span style="color: #66D9EF;">]</span>
<span style="color: #F92672;">return</span> <span style="color: #E6DB74;">`#${toHex(r)}${toHex(g)}${toHex(b)}`</span>
<span style="color: #AE81FF;">}</span>
</pre>
</div>
<p>
Let's test that against this table
</p>
<table id="org9fd0d4c" border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Color</th>
<th scope="col" class="org-left">hsl</th>
<th scope="col" class="org-left">rgb (hex)</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left">Black</td>
<td class="org-left">"0,0,0"</td>
<td class="org-left">#000000</td>
</tr>
<tr>
<td class="org-left">White</td>
<td class="org-left">"0,0,1"</td>
<td class="org-left">#FFFFFF</td>
</tr>
<tr>
<td class="org-left">Red</td>
<td class="org-left">"0,1,.5"</td>
<td class="org-left">#FF0000</td>
</tr>
<tr>
<td class="org-left">Lime</td>
<td class="org-left">"120,1,.5"</td>
<td class="org-left">#00FF00</td>
</tr>
<tr>
<td class="org-left">Blue</td>
<td class="org-left">"240,1,.5"</td>
<td class="org-left">#0000FF</td>
</tr>
<tr>
<td class="org-left">Yellow</td>
<td class="org-left">"60,1,.5"</td>
<td class="org-left">#FFFF00</td>
</tr>
<tr>
<td class="org-left">Cyan</td>
<td class="org-left">"180,1,.5"</td>
<td class="org-left">#00FFFF</td>
</tr>
<tr>
<td class="org-left">Magenta</td>
<td class="org-left">"300,1,.5"</td>
<td class="org-left">#FF00FF</td>
</tr>
<tr>
<td class="org-left">Silver</td>
<td class="org-left">"0,0,.75"</td>
<td class="org-left">#BFBFBF</td>
</tr>
<tr>
<td class="org-left">Gray</td>
<td class="org-left">"0,0,.5"</td>
<td class="org-left">#808080</td>
</tr>
<tr>
<td class="org-left">Maroon</td>
<td class="org-left">"0,1,.25"</td>
<td class="org-left">#800000</td>
</tr>
<tr>
<td class="org-left">Olive</td>
<td class="org-left">"60,1,.25"</td>
<td class="org-left">#808000</td>
</tr>
<tr>
<td class="org-left">Green</td>
<td class="org-left">"120,1,.25"</td>
<td class="org-left">#008000</td>
</tr>
<tr>
<td class="org-left">Purple</td>
<td class="org-left">"300,1,.25"</td>
<td class="org-left">#800080</td>
</tr>
<tr>
<td class="org-left">Teal</td>
<td class="org-left">"180,1,.25"</td>
<td class="org-left">#008080</td>
</tr>
<tr>
<td class="org-left">Navy</td>
<td class="org-left">"240,1,.25"</td>
<td class="org-left">#000080</td>
</tr>
</tbody>
</table>
<div class="org-src-container">
<pre class="src src-js">
<span style="color: #F92672;">return</span> colors.map<span style="color: #AE81FF;">(</span><span style="color: #66D9EF;">(</span><span style="color: #A6E22E;">[</span>color, hsl, rgb<span style="color: #A6E22E;">]</span><span style="color: #66D9EF;">)</span> => <span style="color: #66D9EF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #A6E22E;">[</span><span style="color: #FD971F;">h</span>,<span style="color: #FD971F;">s</span>,<span style="color: #FD971F;">l</span><span style="color: #A6E22E;">]</span> = hsl.split<span style="color: #A6E22E;">(</span><span style="color: #E6DB74;">`,`</span><span style="color: #A6E22E;">)</span>.map<span style="color: #A6E22E;">(</span>parseFloat<span style="color: #A6E22E;">)</span>
<span style="color: #F92672;">return</span> <span style="color: #A6E22E;">[</span>color, hsl, rgb, hslToRgb<span style="color: #E6DB74;">(</span>h, s, l<span style="color: #E6DB74;">)</span><span style="color: #A6E22E;">]</span>
<span style="color: #66D9EF;">}</span><span style="color: #AE81FF;">)</span>
</pre>
</div>
<p>
Yes!
</p>
<p>
Well now it's just a matter of using the above. But also, in the spirit of lispy-ness, since we <b>can</b> make it more flexible lets make it more flexible. I have a feeling it will come in handy. Although the step size is set, why not treat it as a default? We can then pass an optional parameter <b>to</b> the yield and if one is passed we would use <b>that</b> step size
</p>
<div class="org-src-container">
<pre class="src src-web"><article id="gradient-colors-function">
<div style="width:400px;height:200px;"></div>
</article>
<script>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">gradientColors</span> = <span style="color: #F92672;">function</span> * <span style="color: #AE81FF;">(</span>defaultStepSize<span style="color: #AE81FF;">)</span> <span style="color: #AE81FF;">{</span>
<span style="color: #F92672;">let</span> <span style="color: #FD971F;">nextHueProportion</span> = 0
<span style="color: #F92672;">while</span><span style="color: #66D9EF;">(</span><span style="color: #AE81FF;">true</span><span style="color: #66D9EF;">)</span> <span style="color: #66D9EF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">hue</span> = Math.<span style="color: #A6E22E;">floor</span><span style="color: #A6E22E;">(</span>360 * nextHueProportion<span style="color: #A6E22E;">)</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">stepSize</span> = <span style="color: #F92672;">yield</span> <span style="color: #A6E22E;">hslToRgb</span><span style="color: #A6E22E;">(</span>hue, 1, .5<span style="color: #A6E22E;">)</span>
nextHueProportion += stepSize === <span style="color: #AE81FF;">undefined</span> ? defaultStepSize : stepSize
nextHueProportion -= Math.<span style="color: #A6E22E;">floor</span><span style="color: #A6E22E;">(</span>nextHueProportion<span style="color: #A6E22E;">)</span>
<span style="color: #66D9EF;">}</span>
<span style="color: #AE81FF;">}</span>
<span style="color: #F92672;">const</span> <span style="color: #A6E22E;">startGradientFade</span> = <span style="color: #AE81FF;">(</span>element, cycleLengthMilliseconds<span style="color: #AE81FF;">)</span> => <span style="color: #AE81FF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">colors</span> = <span style="color: #A6E22E;">gradientColors</span><span style="color: #66D9EF;">(</span>15.0/cycleLengthMilliseconds<span style="color: #66D9EF;">)[</span>Symbol.iterator<span style="color: #66D9EF;">]()</span> //15 is a fine default frame size, though it can be variable
<span style="color: #F92672;">const</span> <span style="color: #A6E22E;">adjustColor</span> = <span style="color: #66D9EF;">(</span>prevTime<span style="color: #66D9EF;">)</span> => <span style="color: #66D9EF;">{</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">now</span> = Date.<span style="color: #A6E22E;">now</span><span style="color: #A6E22E;">()</span>
element.style.backgroundColor = colors.<span style="color: #A6E22E;">next</span><span style="color: #A6E22E;">(</span><span style="color: #E6DB74;">(</span>now - prevTime<span style="color: #E6DB74;">)</span>/cycleLengthMilliseconds<span style="color: #A6E22E;">)</span>.value
window.<span style="color: #A6E22E;">requestAnimationFrame</span><span style="color: #A6E22E;">(</span><span style="color: #E6DB74;">()</span> => <span style="color: #A6E22E;">adjustColor</span><span style="color: #E6DB74;">(</span>now<span style="color: #E6DB74;">)</span><span style="color: #A6E22E;">)</span>
<span style="color: #66D9EF;">}</span>
<span style="color: #A6E22E;">adjustColor</span><span style="color: #66D9EF;">(</span>Date.<span style="color: #A6E22E;">now</span><span style="color: #A6E22E;">()</span><span style="color: #66D9EF;">)</span>
<span style="color: #AE81FF;">}</span>
<span style="color: #A6E22E;">startGradientFade</span><span style="color: #AE81FF;">(</span>document.<span style="color: #A6E22E;">querySelector</span><span style="color: #66D9EF;">(</span>"#gradient-colors-function"<span style="color: #66D9EF;">)</span>, 5000<span style="color: #AE81FF;">)</span>
</script>
</pre>
</div>
<iframe src="./gradient-colors-function.html" />
</div>
</div>
</div>
</div>
<div id="footnotes">
<h2 class="footnotes">Footnotes: </h2>
<div id="text-footnotes">
<div class="footdef"><sup><a id="fn.1" class="footnum" href="#fnr.1">1</a></sup> <div class="footpara"><p class="footpara">
<a href="http://reilley4color.blogspot.com/2016/05/munsell-hue-circle.html">Image from color blog</a>
</p></div></div>
</div>
</div></div>
<div id="postamble" class="status">
<p class="author">Author: George Mauer</p>
<p class="date">Created: 2020-05-28 Thu 23:54</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
</body>
</html>