using flexbox

Hi, first of all, nice attempt, from user point of view everything (almost) looks OK and usable, page is responsive and nothing in layout is broken. Few thinks I noticed:

• email validation is not correct, valid address like [email protected] is not passing
• For styling inputs in case of error I would recommend use toggling extra class for element. You can set default visibility for error elements as display: none and with some class like .active you can define CSS rule to display: block or whatever you need.
• CSS looks OK for beginner, I recommend you to check at least BEM because one of the hardest part of FE is CSS and keep it organised in larger project.
• there is more space for improvement (use at least TS, SASS or other CSS preprocessor, minify CSS/JS, images optimisation...)

Overall good work, usable for one time job (page is created and code almost never edited/updated), keep up improving your skills, you are on the good way.