Question: I have this triangle in CSS:

```css
.triangle {
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 200px 200px 0 0;
  border-color: #007bff transparent transparent transparent;
}
```

```html
<div class="triangle"></div>
```

How can I apply a 1px shadow on the hypotenuse line?

Answer: 
You can make this *without using borders* by employing an angled gradient.

```css
div {
  width: 200px;
  height: 200px;
  margin: 2em auto;
  background: linear-gradient(to bottom right, #007bff 50%, rgba(0,0,0,.5) 50%, transparent 52%);
}
```

```html
<div></div>
```
Working for me with this code:

```css
.triangle-use-dropshadow{
content: ' ';
    width: 0;
    height: 0;
    border: 14px solid;
    border-color: #fdfdfd transparent transparent transparent;
    filter: drop-shadow(-2px -4px 2px rgba(0,0,0,0.5));
}
```

```html
<div class="triangle-use-dropshadow"></div>
```