Image-free CSS Tooltip Pointers – A Use for Polygonal CSS? 0
A while back, Tantek Celik released A Study of Regular Polygons, which used a little-known CSS trick to create non-rectangular shapes such as triangles and pentagons with nothing more than an ordinary HTML element. The experiment is very interesting and really cool, but the practical takeaway wasn’t immediately clear.
Recently however, we’ve been working on building the markup for upcoming jQuery UI widgets, we came across a case that prompted us to take another look at the technique. The tooltip widget design, fairly common in websites these days, uses a small triangular “speech bubble” pointer that is typically created quite easily with a background image. However, we wanted to maintain our commitment to keeping our jQuery UI widgets entirely themable, and using an image for that purpose wouldn’t fit with the flexibility jQuery UI requires. How to solve this problem without any need for images? That challenge led us to the experimental approach using polygonal CSS, shown below.
The Design Approach
The jQuery UI planning wiki page has several examples for the tooltips we intend to build into the library. For this article though, we’re just focusing the following simple tooltip design:
The challenge in executing this for jQuery UI is that we’ll need to pull it off without relying on images for the triangle shapes, because the jQuery UI CSS Framework enables markup to be entirely re-themable, using an infinite combination of backgrounds, borders, corner radius, and more. Using custom images for the pointer triangles would never match the flexibility and scalability of CSS alone. Since polygonal CSS can create non-rectangular shapes without images, perhaps it can provide the means to achieve our goal.
Polygonal CSS works by setting an element’s width to something small and then setting thick borders on less than 4 sides. To make a triangle shape, 2 of those borders have to have transparent color, essentially masking out the one visible border at an angle in attempt to connect corners.
The concept is easiest explained through a code sample. The following CSS will style a div into a red triangle:
