Container Queries Mixin: Stylesheet version (using $this as a keyword for matching tags)

container-queries-stylesheet-edition.es5.js

function containerQuery(selector, test, stylesheet) {

  var tag = document.querySelectorAll(selector)
  var style = ''
  var count = 0

  for (var i=0; i<tag.length; i++) {

    var attr = (selector+test).replace(/\W+/g, '')

    if (test(tag[i])) {

      var css = stylesheet.replace(/:self|\$this/g, '[data-' + attr + '="' + count + '"]')

      tag[i].setAttribute('data-' + attr, count)
      style += css
      count++

    } else {

      tag[i].setAttribute('data-' + attr, '')

    }

  }

  return style

}

container-queries-stylesheet-edition.es6.js

let containerQuery = (selector, test, stylesheet) => {

  let style = ''
  let count = 0

  document.querySelectorAll(selector).forEach(tag => {

    const attr = (selector+test).replace(/\W+/g, '')

    if (test(tag)) {

      const css = stylesheet.replace(/:self|\$this/g, `[data-${attr}="${count}"]`)

      tag.setAttribute(`data-${attr}`, count)
      style += css
      count++

    } else {

      tag.setAttribute(`data-${attr}`, '')

    }

  })

  return style

}

The ES5 version is a bit incorrect. Template literals are not an ES5 feature so the backticks need to go away.
In the ES6 version, I believe you don't ever change attr and css in the forEach call so they should be consts.