What does DOM reflow occur?

Answer

Reflow is the name of the web browser process for re-calculating the positions and geometries of elements in the document, for the purpose of re-rendering part or all of the document. 

Reflow occurs when:

• Insert, remove or update an element in the DOM.
• Modify content on the page, e.g. the text in an input box.
• Move a DOM element.
• Animate a DOM element.
• Take measurements of an element such as offsetHeight or getComputedStyle.
• Change a CSS style.