I tried almost all tags that comes to my mind:

<table cellspacing="0" cellpadding="0" style="width:100%; text-align:right; border:none; border-collapse:collapse;"><tbody><tr style="border:none;"><td style="width:30%; text-align:right; border:none;"></td></tr>...

and so on, but the effect is always the same - 1px width border for the table :/

What else can I do to change this?
I saw some itch pages with tables without borders, but they're in custom divs - do I need a custom CSS, or is it possible without it?

We have a strict list of approved attributes for HTML tags, and those are not included. You’ll have to use CSS to change table style. You can request it by contacting support.