An HTML file transmitted to a client device is configured to call JavaScript code, where the JavaScript code is configured to call a requestAnimationFrame browser application programming interface (API) with an argument identifying a callback function. The callback function is configurable to cause the web browser to: 1) reset a first Cascading Style Sheets (CSS) animation of a first DOM element, the first CSS animation being configured to cause the first DOM element to transition from a hidden state to a visible state during a first time duration specified by a first animation-duration property of the first element using a specific set of CSS properties; and 2) call the requestAnimationFrame browser API with the argument identifying the callback function. Thus, the first DOM element remains hidden as long as the browser performs frame refreshes at an optimal rate. However, when the browser's main thread is busy, the first DOM element transitions to a visible state.