Cloudflare fast-google-fonts service worker with google amp-custom style tag

Tue, Feb 18, 2020

The service worker template on Cloudflare’s Workers for loading google fonts fater, injects the HTML with font CSS before the page loads. The CSS is embedded in a <style> tag. AMP however does not allow ‘style’ tags. It says: “Only amp-boilerplate and amp-custom ‘style’ tags are allowed, and only in the document head.". Moreover, only one amp-custom ‘style’ tag is permissable.

Here’s the code seen in the fast-google-fonts worker file that replaces the actual CSS.

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// Replace the actual css
    let cssString = "<style" + mediaStr + ">\n";
    cssString += fontCSS;
    cssString += "\n</style>\n";
    content = content.split(matchString).join(cssString);
    fontCSSRegex.lastIndex -= matchString.length - cssString.length;

They are using regex to match the ‘link’ tag and replace it with the generated font CSS. Lets use the same method and find the existing amp-custom ‘style’ tag in the content and inject the CSS there. Replace the code above with:

    const ampCustomRegex = /<style amp-custom>/g;
    let matchTag = ampCustomRegex.exec(content);
    const stringMatch = matchTag[0];
    let cssString = "<style amp-custom" + mediaStr + ">\n";
    cssString += fontCSS;
    content = content.split(stringMatch).join(cssString);
    content = content.split(matchString).join('');
    fontCSSRegex.lastIndex -= stringMatch.length - cssString.length;

The above code finds the <style amp-custom> tag in the content and replaces it with the same tag but followed by the calculated fontCSS. Adding font CSS before the existing custom CSS. Use the original matchString to remove the link tag as before and reset the lastIndex using the new stringMatch.

Putting the @font-face CSS before does not make a difference so all should be good.

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Usman Khwaja

Front end engineer & Jamstack developer

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