HubSpot is a marketing automation tool that offers forms to embed on your website. Here's how to disable the css that comes with it. Add an empty 'css' parameter.

<script charset="utf-8" type="text/javascript" src="//js.hsforms.net/forms/v2.js">
  hbspt.forms.create({
    portalId: "xxxx",
    formId: "xx",
    css: ""
});
</script>