I have a bootstrap project with some other CSS that I inherited. I don't want to change any of the existing styles, but I'd like to add a solid-color child div that is flush with its parent on the top, left and bottom edges, and some number of pixels wide (much narrower than the parent).

The parent's classes which I'd like to keep intact are:

```
.my-heading {
  padding: 10px 15px;
  border-bottom: 1px solid transparent;
  border-top-left-radius: 3px;
  border-top-right-radius: 3px;
}

.my-link {
  background-color: transparent;
  padding: 0;
  margin: 0;
  border: 0;
  position: relative;
}

```

My naive understanding for how to build the child makes me think the CSS should look like:

```
margin-top: 0px;
margin-left: 0px;
margin-bottom: 0px;
width: 40px;
background-color: red;

```

But I still see margins. I've tried several variations changing padding also to no effect. I've seen answers that boil down to `display:table-cell` (couldn't make that work) or to `position:absolute`, which makes the child appear outside the parent.

It seems like this ought to be a simple problem, but I'm out of ideas.