Working on the OptionsBrew.com project (website is powered by Drupal 8) I faced the following responsive table issue while browsing content with a table on smaller screens (phone)

Broken Table on smaller screens

Broken Table on a smaller screen

Luckily there is a simple CSS workaround:

in the .css file add the following to table properties:

   display: block;
   overflow-x: auto;

like this:

table{
    border-collapse: separate;
    border-spacing: 0;
    width:100%;
    margin: 25px 0 40px;
    display: block;
    overflow-x: auto;
}

here is how it looks now

Responsive table

Responsive table

You can see a live example here: Top 20 Most Active Stock Options May 2020