SLS2 inserts your custom HTML inside of an iFrame so you will need to specify the title attribute value. The title attribute should be a brief description of the contents embedded in the iFrame. For example, use "Google Map" if you've embedded a Google Map of your location. This title is for screen readers and is not displayed on the page.
You Own the Content
Keep in mind that any content you add to your site using the Custom HTML Module is part of your site. If any of that content is non-compliant, then your site has an accessibility issue. This is particularly true if you embed content from another site. When in doubt about the accessibility of the source, it may be better to link to that content rather than embed it in your site.