With this Bootstrap icons css link
```
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/font/bootstrap-icons.css">
```
and font name `bootstrap-icons` it worked:
```
var image_style_warning = new ol.style.Style({
  text: new ol.style.Text({
    text: '\uf220',
    fill: new ol.style.Fill({color: 'red'}),
    stroke: new ol.style.Stroke({color: 'yellow', width: 1}),
    font: '100px bootstrap-icons'
  })
});
```
Here is the result:

[![enter image description here][2]][2]

This shows there is a race condition and initially icon is displayed before font is loaded. When icon style is set with some `setTimeout` delay, event with 0 delay value, icon is displayed correctly also on initial page show.


  [1]: https://i.sstatic.net/cVLPJ.png
  [2]: https://i.sstatic.net/3W15a.png