a) async and defer attributes of script tag execute before the DOMContentLoaded event
b) defer executes each script sequentially
c) async executes each script when it is ready
d) all older browsers supports async attribute

Answer: d
Explanation: In async attribute, the script is executed asynchronously with the rest of the page (the script will be executed while the page continues the parsing). Defer, the Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded event. Older browsers doesn’t support async attribute.