CSS grid layout

Looks nice. Some feedback.

• Heading levels should go in order. If you used a h3 for the name, you shouldn't be using a h1 for the testimonial text that comes after the name.
• You should not have multiple h1 elements in a web page. h1 is usually the top most heading that describes the content of the page. In this design you actually don't have a h1. But you can create a visually-hidden element and give that a h1 if you want.
• Looking at how you have used the heading levels, i feel like you are deciding the heading level based on how they are styled in the design. You should not be doing that. You should decide what elements to use based on their semantic meanings. Not styles. After that you can use CSS to style the headings however you want.
• I would use a h2 for each name and then use <p> elements for the testimonial text. I would also wrap the testimonial <p> elements in a <blockquote> element.
• I think it would be better if you give the container a max-width in rem, so that the grid items doesn't keep increasing in size on larger screens.
• It's better to use rem for font-sizes, media queries and max-width. If the user changes their browser font size, text in your site won't respect that if the font-sizes are defined in px. If you have them in rem, the text will scale accordingly with the user's preferred font size. I usually use rem for all values, not only fonts. But it's just preference.