Flex-Height

HTML

<div class="container">
    <span class="main">This is the flex container, height is 20px</span>
    <span class="main">This is not the flex container, height is 19px</span>
</div>

CSS

.container {
    display: flex;
    background-color: yellow;
    border: 1px solid red;
}

.main {
    border: 1px solid black;
}