注意，全角空格将转换为半角空格
<style>
body {
margin: 0;
}
div {
display: flex;
justify-content: space-between;
height: 100%;
background-color: #f2f2f2;
}
textarea {
width: 45%;
border: none;
outline: none;
resize: none;
}
button {
margin: auto 0;
width: 60px;
height: 35px;
border: 2px solid #aaa;
cursor: pointer;
}
</style>
<div>
<textarea id="l"></textarea>
<button></button>
<textarea id="r" readonly></textarea>
</div>
<script>
window.onload = function () {
var l = document.getElementById('l')
var r = document.getElementById('r');
var b = document.querySelector('button');
b.addEventListener('click', function () {
var h = l.value.split('\n');
var a = h.map(i => `<p>${i}</p>`).join('\n');
r.value = a.replace(/ /g, ' ');
r.select();
document.execCommand('copy');
});
};
</script>