Using Bootstrap with Accelerated Mobile Pages

The other day, someone asked me how to use Bootstrap with Accelerated Mobile Pages. My initial reaction was: “Don’t use Bootstrap.” However, it turns out that there are a lot of people using Bootstrap. Some of those users might want to use AMP HTML without rebuilding everything from scratch.

Bootstrap makes it very convenient to build responsive, mobile-first websites. By simply including Bootstrap’s CSS, front-end developers gain access to an extensive grid system and a library of reusable components, but that convenience comes at a cost: bootstrap.min.css adds 118k to your critical path resources. On top of that, it’s also likely that your Bootstrap site doesn’t use all of the included CSS classes.

AMP CSS Restrictions

AMP makes some hard restrictions with CSS: all CSS must be inline and size-bound. This is because CSS blocks the critical rendering path, which means that the web browser will delay rendering content until the CSS Object Model is constructed.

  • Inline CSS allows the CSSOM to be constructed on the initial request so the page can be rendered without waiting for an additional network call. On current mobile networks, latency (and not bandwidth) has become the new performance bottleneck. Mobile devices are constantly switching from high power to low power Radio Resource Control states, which adds additional latency on top of standard internet routing latency.
  • The size-bound ensures that front-end developers don’t go crazy with hyper-specific CSS classes or copy/paste the entirety of bootstrap.min.css into the <head> tag.

Without these restrictions, it would be impossible for AMP to meet its performance goals.

Cleaning Bootstrap’s CSS

It’s possible to use Bootstrap classes with AMP, but you’ll have to remove the unused CSS classes and inline the result in the <style amp-custom> tag. It’s a good practice to prune unused CSS, so this technique can be included in any web development asset pipeline, including Rails or Django.

Since Node is popular these days, let’s look at how to create this pipeline using Gulp.

Remove Unused CSS

purifycss helps us remove unused CSS classes. It can be integrated into Gulp like this:

var purify = require('gulp-purifycss');

gulp.task('purify', function() {
  return gulp.src(SOURCE.BOOTSTRAP_CSS)

Minify and Insert CSS into AMP HTML

For this step, we use clean-css to minify the CSS and then use gulp-html-replace to insert the result into the AMP HTML.

var cleanCSS = require('gulp-clean-css');
var htmlreplace = require('gulp-html-replace');

gulp.task('inline-css', function() {
  return gulp.src(SOURCE.AMPHTML)
      'cssInline': {
        'src': gulp.src(SOURCE.CLEANED_CSS).pipe(cleanCSS()),
        'tpl': '<style amp-custom>%s</style>'

Validate the Output

Since AMP disallows specific styles from being used, we can run the amphtml-validator to find potential problems in the final output.

var amphtmlValidator = require('amphtml-validator');
var fs = require('fs');

gulp.task('validate', function() {
  amphtmlValidator.getInstance().then(function (validator) {
    var input = fs.readFileSync(BUILD_PATH + '/index.html', 'utf8');
    var result = validator.validateString(input);
    // do something with validation result

Validation is an important part of the build process because we need to ensure that our CSS creates the desired layout and styling while also adhering to the AMP spec.

Running the amphtml-validator finds some invalid Bootstrap classes:

  • CSS syntax error in tag ‘style amp-custom’ - saw invalid at rule ‘@-ms-viewport’
  • The text (CDATA) inside tag ‘style amp-custom’ contains ‘CSS !important’, which is disallowed

After removing these classes from the source CSS, we have valid AMP HTML.


Once the unused (and invalid) classes are stripped from the original Bootstrap CSS, the minified output shrinks from 120kb down to 12kb.



The pages look exactly the same and now you have both valid AMP HTML and cleaner CSS.

For reference, here’s the source code for this example.