I finished this but I still get a little triangle error

This is my CSS code but when I hover over the first line with the border there is a triangle error that says “Using height with border can some times make elements larger than you expect.” Can someone please explain why this happens?

 /*Add your CSS below!*/
    td {
        height: 50px;
        border: 1px dashed blue;
    }
    table {
        border: 1px solid black;   
    }