Shuhei Kagawa

Responsive images with a static site generator

Jan 26, 2020 - Blog, JavaScript

Responsive images

An img without width/height attributes causes a page jump when it's loaded. It happens because the browser doesn't know the dimensions of the image until its data is loaded—only the first part that contains dimensions is enough though. It's common to specify width and height of img tag to avoid the jump.

<img src="/images/foo.jpg" alt="Foo" width="800" height="600" />

But img tag with width and height doesn't always work well with Responsive Design because the dimensions are fixed. I wanted images to fit the screen width on mobile phones. So I left images without width/height and let them cause page jumps.

Recently, I came across a similar problem at work and learned a cool technique to create a placeholder of the image's aspect ratio with padding-top. If the image's aspect ratio is height/width = 75/100:

<div style="position: relative; padding-top: 75%;">
  <img style="position: absolute; top: 0; left: 0; max-width: 100%;" />

The div works as a placeholder with the image's aspect ratio that fits the width of its containing element. Because the img tag has position: absolute, it doesn't cause a page jump when it's loaded.

I decided to implement it on this blog. This blog is made with a custom static site generator. I'm not sure if it's useful for anyone else, but I write how I did it anyway…

Limiting overstretch

In addition to images that are wide enough to always fill the full width of the content area, I had images that are not wide enough to fill the full width of a laptop, but wide enough to fill the full width of a mobile phone. Not to stretch the image on laptops, I decided to go with another wrapper to limit the maximum width of the image. If the image's width is 500px:

<div style="max-width: 500px;">
  <div style="position: relative; padding-top: 75%;">
    <img style="position: absolute; top: 0; left: 0; max-width: 100%;" />

Getting image dimensions

The placeholder technique requires image dimensions. I used image-size module to get image dimensions.

The following function gets dimensions of images in a directory and returns them as a Map.

const util = require("util");
const path = require("path");
const { promises: fs } = require("fs");
const sizeOf = util.promisify(require("image-size").imageSize);

async function readImageSizes(dir) {
  const files = (await fs.readdir(dir)).filter(f => !f.startsWith("."));
  const promises = file => {
    const filePath = path.resolve(dir, file);
    const dimensions = await sizeOf(filePath);
    return [file, dimensions];
  const entries = await Promise.all(promises);
  return new Map(entries);

Custom renderer of Marked

This blog's posts are written in Markdown, and its static site generator uses marked to convert Markdown into HTML. One of my favorite things about marked is that we can easily customize its behavior with a custom renderer.

I used span tags to wrap img tag because they are rendered in p tag, which can't contain block tags like div.

class CustomRenderer extends marked.Renderer {
  image(src, title, alt) {
    const dimensions = this.imageDimensions && this.imageDimensions.get(src);
    if (dimensions) {
      const { width, height } = dimensions;
      const aspectRatio = (height / width) * 100;
      return (
        `<span class="responsive-image-wrapper" style="max-width: ${width}px;">` +
        `<span class="responsive-image-inner" style="padding-top: ${aspectRatio}%;">` +
        `<img class="responsive-image" src="${src}" alt="${alt}">` +
        "</span>" +
    return super.image(src, title, alt);

  // To set images dimensions when images are changed
  setImageDimensions(imageDimensions) {
    this.imageDimensions = imageDimensions;

And CSS:

.responsive-image-wrapper {
  display: block;
.responsive-image-inner {
  display: block;
  position: relative;
.responsive-image {
  position: absolute;
  top: 0;
  left: 0;


Here are a few examples:

Yay, no more page jump! Well, web fonts still make the page slightly jump, but that's another story...

I skipped some details that are specific to my website. The full code is on GitHub.