CSS issue header and footer do not keep their height - html

Question

In the following code, I need to create a box with header and footer with a specific height the central div should be at height 100%.
Currently if I am using flex on #card the heights for header and footer are shorter.
If I remove the flex css from #card it works as expected.
I am aware that removing the flex would solve my issue, but I am interested to know why it is happening and how I could fix it using still flex.
Thanks for your feedback.
#card {
width: 300px;
height: 400px;
background-color: grey;
display:flex;
flex-direction: column;
}
#header {
width: 100%;
height: 50px;
background-color: red;
}
#content {
width: 100%;
height: 100%;
background-color: yellow;
overflow:auto;
}
#footer {
width: 100%;
height: 100px;
background-color: blue;
}
<div id="card">
<div id="header">
</div>
<div id="content">
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
</div>
<div id="footer">
</div>
</div>

Answer 1

Remove the height:100% and use flex grow:1 and then change the heights to min-height (sometimes I like to use both min and max to ensure the height is fixed)
#card {
width: 300px;
height: 400px;
background-color: grey;
display: flex;
flex-direction: column;
}
#header {
width: 100%;
min-height: 50px;
background-color: red;
}
#content {
width: 100%;
flex-grow:1
background-color: yellow;
overflow: auto;
}
#footer {
width: 100%;
min-height: 100px;
background-color: blue;
}
<div id="card">
<div id="header">
</div>
<div id="content">
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has
survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing
software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took
a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets
containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's
standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
</div>
<div id="footer">
</div>
</div>
Update - removing flex shrink per #Michael_B comment
#card {
width: 300px;
height: 400px;
background-color: grey;
display: flex;
flex-direction: column;
}
#header {
width: 100%;
height: 50px;
flex-shrink: 0;
background-color: red;
}
#content {
width: 100%;
flex-grow:1
background-color: yellow;
overflow: auto;
}
#footer {
width: 100%;
height: 100px;
flex-shrink: 0;
background-color: blue;
}
<div id="card">
<div id="header">
</div>
<div id="content">
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has
survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing
software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took
a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets
containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's
standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
</div>
<div id="footer">
</div>
</div>

Answer 2

This is because of your height element on your header and footer tags.
This CSS would allow you to use the display: flex; without altering the header and footer height. 100% height works when the parent has a well defined height.
#card {
width: 300px;
height: 400px;
background-color: grey;
display: flex;
flex-direction: column;
}
#header {
width: 100%;
max-height: 50px;
height: 100%;
background-color: red;
}
#content {
width: 100%;
height: 100%;
background-color: yellow;
overflow: auto;
}
#footer {
width: 100%;
max-height: 100px;
height: 100%;
background-color: blue;
}
<div id="card">
<div id="header">
</div>
<div id="content">
Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has
survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing
software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took
a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets
containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's
standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
</div>
<div id="footer">
</div>
</div>

Answer 3

It's running right; you set it to height: 400px and the height is 400px (inspect it).
I think what you're looking for is to replace height: 400px with height: 100vh to make #card cover the entire height of any screen.