Your previous approach is longer supported because Squarespace tightened up their security to filter out code from various locations, including the tagline. As your code is removed before the page is rendered, it no longer appears on the page.
As an alternative, you can either:
insert your HTML using Code Injection (Settings > Advanced > Code Injection) and modify the styling to position it absolutely where you want it to appear, or
insert it into the correct position in the DOM using some JS.
dauray reacted to paul2009 in 7.0 (Mercer) – Replacing tagline with image
