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: 
Since `box-shadow` won't work, you have to apply a drop shadow filter on the triangle:

```css
.triangle {
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 200px 200px 0 0;
  border-color: #007bff transparent transparent transparent;
  -webkit-filter: drop-shadow(1px 1px 1px rgba(0,0,0,.5));
  filter: drop-shadow(1px 1px 1px rgba(0,0,0,.5));
}
```

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