Yeah this is tricky! I think our Less support story is currently lagging behind Sass’. But I’d recommend having something in your snowpack.config.js like (run npm install @snowpack/plugin-run-script first):

module.exports = {
  plugins: [
    ['@snowpack/plugin-run-script', { cmd: 'less [input.less] [output.css] …' }]
  ]
}

And in your HTML:

+   <link rel="stylesheet" href="[output.css]" />
  </head>

Basically what this does is handles Less using more of its CLI and ecosystem, less embedded within Snowpack. You may experience less weird behavior.

We try and discourage plugin-run-script for normal usage, and especially when we have an existing snowpack plugin for something (e.g. Sass, which we now encourage you to use @snowpack/plugin-sass). But until we have a @snowpack/plugin-less story I believe plugin-run-script may be the easiest/most reliable method.