Yes, it is important! Affects
- to download and display speed
- to display in different browsers
- to promote the site in search
Check your site errors in this resource - https://validator.w3.org/nu/
This resource only shows errors in the HTML of the site.
If you have errors JS, CSS, PHP and others, then I will issue a separate individual kwork.
Look attentively at the last mistake. If there is an inscription "Fatal Error", it means that there are more errors on your site, but after the last one, it is simply impossible to count all the others.
If you fix the errors on the main page, then on other pages there will be less of them. For a complete correction, you need to edit all pages of the site.
Having corrected the errors of one product, it is likely that all other products will be lost, since they use the same template.
Sometimes (though rarely) correcting an error can entail a large chain of rework. Usually, this is related to ... ... however, you do not need to delve into the nuances now. If I do not correct some mistake, I will give a full explanation of why I had to leave it.
You decide, fix only the critical errors "Error" or all errors together with the warnings "Warning"