Ultimately you cannot really 'secure' your HTML code in a way that it is impossible to read it with the Developer Tools because the client browser still has to be able to read it. You can try little things to make it more difficult, for example, obfuscation.
The only way to secure sensitive data is with a proper, back-end application that can interact with the sensitive data and deliver them to the front-end in a secure manner.
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Ultimately you cannot really 'secure' your HTML code in a way that it is impossible to read it with the Developer Tools because the client browser still has to be able to read it. You can try little things to make it more difficult, for example, obfuscation.
The only way to secure sensitive data is with a proper, back-end application that can interact with the sensitive data and deliver them to the front-end in a secure manner.