H1 width style using Gutenberg in CHROME

This might be an elementary CSS question…

1 – I add a Gutenberg heading element
2 – I add my text
3 – I want the element (H1) to fill 90% of available space.
4 – I add a class to the H1 element Additional CSS Class
h1.abc {max-width:90%;}

5 – It works in FIREFOX and Microsoft Edge.
6 – It does NOT work in Chrome.
Any idea why?