The main files:
home.html (access: read only)
...
    <h1>Heading 1</h1>
...
externalFile.css (access: read only)
.tagh1 {
  margin: 10px 0;
  font-family: inherit;
  font-weight: bold;
}
myfile.css (access: read/write)
h1 {
  color: #08C;
}
How I can include the styles of .tagh1 (externalFile.css) in the h1 CSS rule (myfile.css), without copying and pasting them from one file to the other?
Logical answer:
h1 {
  color: #08C;
  /* Just copy and paste them */
  margin: 10px 0;
  font-family: inherit;
  font-weight: bold;
}
but I cant do this :)
Edit:
I thought it might be something like this:
.tag1 {
  property1: value1;
  property2: value2;
}
.tag2 {
  include(.tag1); /* or something */
  property3: value3;
}
To avoid repeating the code.