After reviewing your question, the regex code provided below should be suitable
var regex = /((<style>)|(<style type=.+)|(<<\/style>))/g;
var str = `<style>
styles
</style>
<style type="text/css">
styles
</style>`;
const subst = ``;
// The substituted value will be stored in the result variable
var result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Check out the demo here
Update
Take a look at the updated regex which removes both the styles and the content within them.
var regex = /((<style>)|(<style type=.+))((\s+)|(\S+)|(\r+)|(\n+))(.+)((\s+)|(\S+)|(\r+)|(\n+))(<<\/style>)/g;
const str = `<style>
styles
</style>
<style type="text/css">
styles
</style>
non html content`;
var subst = ``;
// The substituted value is saved in the result variable
var result = str.replace(regex, subst);
console.log('Substitution result: ', result);
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