you can use title attribute:

<button variant="contained" title="i apply!">apply</button>
<button variant="contained" title="i delete!">remove</button>

for making bold when clicked, you have many ways. here is the simple css solution:

<button variant="contained" title="i apply!" classname="apply-btn">apply</button>

then write css like this:

.apply-btn:focus {
font-weight: bold;

notice that you should write your css in a separate file. or you can use usestyles if you want to leverage css-in-js.

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