パーセント指定

[Item-C ]
box-sizing: border-box;
padding: 0 10%;
width: 100%;

[Item-C/ Core]
width: 100%

[Item-C]
padding-left: 10%

[Item-C]
padding-right: 10%

[Item-D]
box-sizing: content-box;
padding: 0 10%;
width: 100%;

[Item-D/ Core]
width: 100%

[Item-D]
padding-left: 10%

[Item-D]
padding-right: 10%

[Item-A ]
box-sizing: border-box;
width: 100%;

[Item-A/ Side]
width: 10%

[Item-A/ Core]
width: 80%

[Item-A/ Side]
width: 10%

[Item-B]
box-sizing: content-box;
width: 100%;

[Item-B/ Side]
width: 10%

[Item-B/ Core]
width: 80%

[Item-B/ Side]
width: 10%