Responsive chat app illustration using CSS flexbox

It was a project of intermediate difficulty and there were several challenges I faced such as:

1. Building the oval graphics at the starting and ending of the page and making it responsive.

2. Creating the phone outline. First, I went with a border but that didn't work out. So, the next approach was with an outline. However, using the color white, it was indistinguishable from the page background. Finally, using a box-shadow did the job.

3. The responsiveness of the main hero area that includes the chat and the description text took some time to achieve.

Feedback requested:

1. Adherence to semantic HTML
2. Adherence to BEM
3. Any other general advice that would make the code better.