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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <h1>&lt;selectmenu&gt; plain </h1>
<selectmenu>
    <optgroup label="Optgroup 1">
        <option>Option 1</option>
        <option>Option 2</option>
        <option>Option 3</option>
        <option>Option 4</option>
        <option>Option 539845734587982374</option>
    </optgroup>
    <optgroup label="Optgroup 2">
        <option>Option 1</option>
        <option>Option 2</option>
        <option>Option 3</option>
        <option>Option 4</option>
        <option>Option 5</option>
    </optgroup>
</selectmenu>

<span><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br></span>
<h1>&lt;selectmenu&gt; styled</h1>
<selectmenu class="styled">
    <optgroup label="Optgroup 1">
        <option>Option 1</option>
        <option>Option 2</option>
        <option>Option 3</option>
        <option>Option 4</option>
        <option>Option 539845734587982374</option>
    </optgroup>
    <optgroup label="Optgroup 2">
        <option>Option 1</option>
        <option>Option 2</option>
        <option>Option 3</option>
        <option>Option 4</option>
        <option>Option 5</option>
    </optgroup>
</selectmenu>

<span><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br></span>

<h1>&lt;selectmenu&gt; filled with lots of strange stuff</h1>
<p>Link, SVG image, table, input, another select menu, datepicker, YouTube Live stream embed</p>
<selectmenu class="filled">
    <optgroup label="Optgroup 1">
        <option><a href="https://wwsiv.de">WWSIV</a></option>
        <option>SVG <br>
            <svg style="width: 100px;" viewBox="0 0 235.15756 223.20731"><g id="wwsiv_svg_logo"><linearGradient id="ee_rainbow" x1="0" x2="0" y1="0" y2="1" gradientUnits="objectBoundingBox"><stop offset="0" stop-color="#F00"></stop><stop offset="0.1667" stop-color="#F00"></stop><stop offset="0.1667" stop-color="#FF9800"></stop><stop offset="0.3333" stop-color="#FF9800"></stop><stop offset="0.3333" stop-color="#FF0"></stop><stop offset="0.5" stop-color="#FF0"></stop><stop offset="0.5" stop-color="#009800"></stop><stop offset="0.6667" stop-color="#009800"></stop><stop offset="0.6667" stop-color="#00F"></stop><stop offset="0.8333" stop-color="#00F"></stop><stop offset="0.8333" stop-color="#980098"></stop><stop offset="1.00" stop-color="#980098"></stop></linearGradient><linearGradient id="ee_bi" x1="0" x2="0" y1="0" y2="1" gradientUnits="objectBoundingBox"><stop offset="0" stop-color="#D60270"></stop><stop offset="0.4" stop-color="#D60270"></stop><stop offset="0.4" stop-color="#9B4F96"></stop><stop offset="0.6" stop-color="#9B4F96"></stop><stop offset="0.6" stop-color="#0038A8"></stop><stop offset="1" stop-color="#0038A8"></stop></linearGradient><linearGradient id="ee_trans" x1="0" x2="0" y1="0" y2="1" gradientUnits="objectBoundingBox"><stop offset="0" stop-color="#5BCEFA"></stop><stop offset="0.2" stop-color="#5BCEFA"></stop><stop offset="0.2" stop-color="#F5A9B8"></stop><stop offset="0.4" stop-color="#F5A9B8"></stop><stop offset="0.4" stop-color="#FFFFFF"></stop><stop offset="0.6" stop-color="#FFFFFF"></stop><stop offset="0.6" stop-color="#F5A9B8"></stop><stop offset="0.8" stop-color="#F5A9B8"></stop><stop offset="0.8" stop-color="#5BCEFA"></stop><stop offset="1" stop-color="#5BCEFA"></stop></linearGradient><linearGradient id="ee_enby" x1="0" x2="0" y1="0" y2="1" gradientUnits="objectBoundingBox"><stop offset="0" stop-color="#FCF434"></stop><stop offset="0.25" stop-color="#FCF434"></stop><stop offset="0.25" stop-color="#FCFCFC"></stop><stop offset="0.5" stop-color="#FCFCFC"></stop><stop offset="0.5" stop-color="#9C59D1"></stop><stop offset="0.75" stop-color="#9C59D1"></stop><stop offset="0.75" stop-color="#2C2C2C"></stop><stop offset="1" stop-color="#2C2C2C"></stop></linearGradient><linearGradient id="ee_fluid" x1="0" x2="0" y1="0" y2="1" gradientUnits="objectBoundingBox"><stop offset="0" stop-color="#ff75a2"></stop><stop offset="0.2" stop-color="#ff75a2"></stop><stop offset="0.2" stop-color="#ffffff"></stop><stop offset="0.4" stop-color="#ffffff"></stop><stop offset="0.4" stop-color="#be18d6"></stop><stop offset="0.6" stop-color="#be18d6"></stop><stop offset="0.6" stop-color="#000000"></stop><stop offset="0.8" stop-color="#000000"></stop><stop offset="0.8" stop-color="#333ebd"></stop><stop offset="1" stop-color="#333ebd"></stop></linearGradient><g fill="#f00" style="isolation:isolate" transform="matrix(0.34253886,0,0,0.34253886,484.41073,89.249015)"><path d="m -738.19378,106.12091 c -21.6769,37.5455 -69.6875,50.40989 -107.23299,28.73299 -37.54549,-21.6769 -50.4099,-69.68749 -28.73299,-107.232989 21.6769,-37.5455011 69.6875,-50.409894 107.23299,-28.7329941 37.54549,21.6769001 50.4099,69.6874931 28.73299,107.2329931" style="isolation:isolate;fill:var(--yellow);fill-opacity:1;stroke-width:4.36111116"></path><ellipse transform="rotate(30)" ry="11.993056" rx="10.902778" cy="454.45862" cx="-690.90076" style="isolation:isolate;fill:#664500;stroke-width:4.36111116"></ellipse><ellipse transform="rotate(30)" ry="15.263889" rx="10.902778" cy="454.45862" cx="-638.5675" style="isolation:isolate;fill:#664500;stroke-width:4.36111116"></ellipse><path d="m -835.4723,22.557459 c 0.55664,1.207676 1.65331,2.142989 3.04814,2.439672 18.76827,4.012396 25.33175,17.415119 25.60241,17.984319 1.03058,2.166175 3.62181,3.10324 5.79499,2.08679 2.17308,-1.016501 3.12094,-3.591439 2.11633,-5.767793 -0.33533,-0.727396 -8.49531,-17.871894 -31.68691,-22.831676 -2.35727,-0.504887 -4.67483,0.99725 -5.17374,3.352984 -0.20986,0.956495 -0.0795,1.908011 0.29878,2.735704 z m 91.17609,41.808581 c -1.38989,-0.80245 -2.40983,-2.20206 -2.67046,-3.91363 -1.45049,-9.655085 -9.0691,-21.788653 -12.90265,-24.001953 -2.4247,-1.3999 -9.25686,-0.756857 -13.25396,0.243922 -2.92433,0.738888 -5.88287,-1.044768 -6.61262,-3.9588 -0.73348,-2.916182 1.03073,-5.875882 3.94163,-6.617494 2.30535,-0.582594 14.16115,-3.280438 21.3786,0.886562 7.57262,4.37205 16.43294,19.841115 18.23604,31.824873 0.45036,2.97934 -1.60347,5.75168 -4.57688,6.20037 -1.27583,0.19501 -2.51234,-0.0707 -3.53962,-0.6638 z" style="isolation:isolate;fill:#664500;stroke-width:4.36111116"></path><path d="m -784.12557,110.09791 c -0.32123,-0.99621 -1.3041,-1.61404 -2.33507,-1.49414 -0.21052,0.0244 -21.13638,2.40068 -39.18209,-8.01802 -18.00034,-10.3925 -26.45452,-29.73124 -26.53486,-29.92368 -0.41168,-0.95274 -1.44252,-1.48748 -2.4615,-1.27516 -1.01737,0.21833 -1.74408,1.12824 -1.72436,2.16689 0.0184,1.09332 0.71741,26.8168 24.17908,40.3624 23.46167,13.5456 46.09217,1.29142 47.0444,0.7585 0.91155,-0.50604 1.33782,-1.58436 1.0144,-2.57679 z" style="isolation:isolate;fill:#664500;stroke-width:4.36111116"></path></g><g fill="#00f" transform="matrix(0.34269805,0,0,0.34269805,724.82375,-370.41153)" style="isolation:isolate"><g><path d="m -2072.1776,1561.8888 -42.8731,-481.0203 h 471.6117 l -42.8732,481.0203 -193.4525,54.3783" style="isolation:isolate;fill:var(--pink);fill-opacity:0.6;stroke-width:4.15038109"></path><path id="logo_shield" d="m -1990.1298,1637.6025 -42.8731,-481.0207 h 471.6117 l -42.8731,481.0207 -192.9327,54.3779 z" style="isolation: isolate; fill: var(--pink); fill-opacity: 1; stroke: var(--purple); stroke-width: 77.4075; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 4; stroke-dasharray: none; stroke-opacity: 1; paint-order: stroke markers;"></path><path d="m -1797.197,1650.1521 155.8097,-42.8718 36.5977,-412.0087 h -192.4074" style="isolation:isolate;fill:#ffffff;fill-opacity:0.23137255;stroke-width:4.15038109"></path><g transform="matrix(1.0460428,0,0,1.0460428,82.747235,-50.818143)"><path id="logo_W_left" style="isolation: isolate; fill: rgb(235, 235, 235); stroke-width: 3.9677; fill-opacity: 1;" d="m -1882.911,1248.4688 -56.2317,-0.248 24.9918,287.9036 57.7395,14.8956 59.2144,-151.4932 v -83.7508 l -13.3741,-0.064 -55.7704,121.8532 z"></path><path id="logo_W_right" style="isolation:isolate;fill:#ffffff;fill-opacity:1;stroke-width:3.96769691" d="m -1711.4806,1248.4688 56.2309,-0.248 -24.9938,287.9036 -57.7375,14.8956 -59.216,-151.4932 v -83.7508 l 13.3831,-0.064 55.7708,121.8532 z"></path></g></g></g></g></svg></option>
        <option>HTML<br>
            <h1>headline</h1>
            <ul>
                <li>List item</li>
                <li>List item</li>
            </ul>
        </option>
        <option><input type="text"></option>
        <option>
            <selectmenu>
                <optgroup label="Optgroup 1">
                    <option>Option 1</option>
                    <option>Option 2</option>
                    <option>Option 3</option>
                    <option>Option 4</option>
                    <option>Option 539845734587982374</option>
                </optgroup>
                <optgroup label="Optgroup 2">
                    <option>Option 1</option>
                    <option>Option 2</option>
                    <option>Option 3</option>
                    <option>Option 4</option>
                    <option>Option 5</option>
                </optgroup>
            </selectmenu>
        </option>
        <option>Option 539845734587982374</option>
    </optgroup>
    <optgroup label="Optgroup 2">
        <option>INPUT DATE<input type="date"></option>
        <option>YOUTUBE VIDEO <iframe width="560" height="315" src="https://www.youtube-nocookie.com/embed/jfKfPfyJRdk" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe></option>
        <option>RANGE INPUT
            <input type="range" id="volume" name="volume"
         min="0" max="11"></option>
        <option>Option 4</option>
        <option>Option 5</option>
    </optgroup>
</selectmenu>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>BOO!
!

CSS 

              
                :root {
    --rainbow-bg: linear-gradient(#f00, #ff0, #0f0, #0ff, #00f, #f0f, #f00);
}

.styled {
    font-size: 20px;
    width: 500px;
    height: 60px;
    border-radius: 10px;
}

.styled::part(button) {
    width: 100%;
    height: 100%;
    padding: 1rem;
    background-color: peachPuff;
    font-size: 40px;
    border-radius: 10px;
    color: white;
}

.styled::part(selected-value) {
    background: red;
    padding: 1rem;
}

.styled::part(listbox) {
    background: black;
    padding: 1rem;
    border-radius: 100px;
    min-height: 200px;
} 

.styled, .styled * {
    background: var(--rainbow-bg);
}

.styled *:focus {
    background: #fff;
}

.filled {
    font-size: 20px;
    width: 500px;
    height: 200px;
    border-radius: 10px;
}

.filled::part(listbox) {
    max-height: 70vh;
}

:where(.styled, .filled) optgroup {
    text-align: center;
}

/************************************************************/
body {
  background: #222;
  height: 100vh;
  font-family: sans-serif;
  color: white;
  line-height: 1.5;
  letter-spacing: 1px;
  margin-top: 2rem;
  max-width: 500px;
  margin: 0 auto;
}

* {
    box-sizing: border-box;
}
!

JS
!
999px

Console