Cap the width of SVG and other image files?

[hamishwillee]

When removing some inline styles as part of preparation for markdown I noticed that once the styles were removed the images spread full page - wider than the text content. To fix this particular case put in width attribute on the img tag, but we really don't want to do that for markdown.

So I think we need some way to constrain image size to width of content "by default". See here for the case:
mdn/content#5657 (comment)

Note it is fairly typical of SSGs to fix the width of text and img content to the same width and then allow users to select an image to zoom it full screen.