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HTML

              
                <main>
    <x-emoji>🏫</x-emoji>
    <x-emoji>🌲</x-emoji>
    <x-emoji>🚌</x-emoji>
</main>
              
            
!

CSS

              
                x-emoji {
  background-color: white;
  display: block;
  margin: 10px;
}

              
            
!

JS

              
                class Util {

    /**
     * Renders the given emoji text at that given font size on a canvas. Returns that canvas.
     * 
     * @param {string} emojiText The emoji text to render.
     * @param {number} width The desired width of the canvas.
     * @param {number} height The desired height of the canvas.
     * @param {boolean} flip True to horizontally flip the image; otherwise false.
     * 
     * @returns The created canvas with the rendered emoji text at the given font size.
     */
    static renderEmoji(emojiText, width, height, flip=false) {
        let size = Math.max(width, height);
        let canvas = document.createElement('canvas');
        canvas.height = size + (size / 8) + 20;
        canvas.width = size * 1.4 + 10;
        let ctx = canvas.getContext('2d');
        ctx.font = `${size}px 'Segoe UI Emoji'`;
        ctx.textBaseline = 'bottom';
        ctx.fillText(emojiText, 5, canvas.height - 5);

        // On Windows, this reduces the thick black edges.
        let [minX, minY, maxX, maxY] = Util.reduceEdges(ctx, 0, 0);
        
        // Redraw the canvas, so that we can remove white space and add a shadow.
        let emojiCanvas = document.createElement('canvas');
        let newWidth = ((maxX - minX) + 3);
        let newHeight = ((maxY - minY) + 3);
        emojiCanvas.width = width;
        emojiCanvas.height = height;
        let emojiCtx = emojiCanvas.getContext('2d');
        if (newWidth > 0 && newHeight > 0) {
            emojiCtx.shadowColor = "black";
            emojiCtx.shadowBlur = 3;
            if (flip) {
                emojiCtx.translate(width, 0);
                emojiCtx.scale(-1, 1);
            }
            emojiCtx.drawImage(
                canvas,
                minX-1, minY-1, newWidth, newHeight,
                0, 0, width, height,
            );
        }

        return emojiCanvas;
    }

    /**
     * For the image data in the given canvas context, applies a fill algorithm
     * to trim off the thick black edges that Windows emojis have around them.
     * On other operating systems, this will hopefully do nothing, but lets 
     * see.
     * 
     * This method also has the additional function of determining where the 
     * left/right/top/bottom edges of the rendered emoji are, which it returns.
     * 
     * @param {CanvasRenderingContext2D} ctx 
     * @param {number} startX 
     * @param {number} startY 
     */
    static reduceEdges(ctx, startX, startY) {
        let { width, height } = ctx.canvas;
        let dataWidth = (width << 2);
        let imgData = ctx.getImageData(0, 0, width, height);
        let visitMap = new Uint32Array(imgData.data.length);
        let queue = [[startX, startY]];
        let [minX, minY] = [width, height];
        let maxX = 0;
        let maxY = 0;

        while (queue.length) {
            // Get pixel position to test from queue.
            let [x, y] = queue.shift();
            let pos = ((y * dataWidth) + (x << 2));

            // Check if we've been here before.
            if (visitMap[pos]) continue;

            visitMap[pos] = 1;

            let [red, green, blue, alpha] = imgData.data.slice(pos, pos + 4);
            let brightness = Math.round(Math.sqrt((red * red * 0.299) + (green * green * 0.587) + (blue * blue * 0.114)));
            let spread = false;

            if (alpha == 0) {
                spread = true;
            }
            else if ((red == 0) && (green == 0) && (blue == 0)) {
                if (Util.WIN) {
                    imgData.data[pos + 3] = 0;
                    spread = true;
                }
            }
            else if (brightness < 70) {
                if (Util.WIN) {
                    imgData.data[pos + 3] = Math.round(brightness * (255 / 70));
                    spread = true;
                }
            }

            if (spread) {
                if (x > 0) queue.push([x - 1, y]);
                if (x < width - 1) queue.push([x + 1, y]);
                if (y > 0) queue.push([x, y - 1]);
                if (y < height - 1) queue.push([x, y + 1]);
            }

            if (imgData.data[pos + 3]) {
                if (x < minX) minX = x;
                if (x > maxX) maxX = x;
                if (y < minY) minY = y;
                if (y > maxY) maxY = y;
            }
        }

        ctx.putImageData(imgData, 0, 0);

        return [minX, minY, maxX, maxY];
    }
}

Util.WIN = navigator.platform.indexOf('Win') > -1;

document.querySelectorAll('x-emoji').forEach(emoji => {
  let emojiText = emoji.textContent;
  emoji.innerHTML = '';
  emoji.appendChild(Util.renderEmoji(emojiText, 300, 300));  
});

              
            
!
999px

Console