There is not. Would be a good addition, I will make a note of it. For now you can set a custom template tag in your script, then use it in the HTML Preview step that follows it. Sample:
Then in subsequent steps in the action the
[[content]] tag is available. I’ve used this technique in this Markdown Preview with iCloud CSS example action, which loads a CSS file from iCloud Drive and inserts it in the preview. Great trick if you have a blog/site CSS file you edit elsewhere and want a preview that matches.