When you change the font-weight of a font, the text will typically cause a bit of a layout shift. That’s because bold text is often larger and takes up more space. Sometimes that doesn’t matter, like a vertical stack of links where the wider/bolder text doesn’t push anything anyway. Sometimes it does matter, like a horizontal row where the wider/bolder text pushes other elements away a smidge.

Ryan Mulligan demonstrates:

Bolding text on mouse hover causes a layout shift that’s especially noticeable when elements start wrapping. Here’s a nifty trick: add a hidden pseudo element with the same text string but set it to the bold font size 🙌

See it on @CodePen: https://t.co/kBzZXqqtmi pic.twitter.com/kdZBTLQ0RD

— Ryan Mulligan (@hexagoncircle) July 20, 2020

Ryan’s technique is very clever. Each item in the list has a pseudo-element on it with the exact text in the link. That pseudo-element is visually hidden, but pre-bolded and still occupies width. So when the actual link text is bolded, it won’t take up any additional width.

It also sorta depends on how you’re doing the layout. Here, if I force four columns with CSS grid and text that doesn’t really challenge the width, the bolding doesn’t affect the layout either:

But if I were to, say, let those links flow into automatic columns, we would have the shifting problem.