Change the DOCTYPE of HTML files in Gatsby. By default, Gatsby adds <!DOCTYPE html> to the start to every HTML file it generates. You can now edit the html portion.
npm install gatsby-plugin-doctypeInclude the plugin in your gatsby-config.js file and use the doctype option.
module.exports = {
plugins: [
{
resolve: `gatsby-plugin-doctype`,
options: {
doctype: 'HTML',
},
},
],
}If you want to use something like HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd" you'll need to make sure that you wrap the option with single quotes/backticks or escape the quotes.
Run a gatsby build as this plugin only changes files with the onPostBuild lifecycle.
You can see the finished result in the HTML files inside public.