Spellcheck support is a built-in feature within the browsers:

These browsers will automatically flag any unrecognized words, usually with a red underline. When you right-click on an underlined word, you'll see a menu of options. You can then choose to correct the word or add it to your dictionary.

If you can't upgrade your browser, you may be able to install a browser add-on to provide spellcheck support. You can check this with your browser provider.

While browsers have spellcheck enabled by default, it's possible to turn it off for: