There are two different objects here. In HTML, you have an <input> element with a "value" attribute [0]. An <input> element also has an internal value, which is initialized using the "value" attribute.
In JavaScript, you have an HTMLInputElement with a "value" property [1]. Getting the "value" property of an HTMLInputElement reads from its <input> element's internal value, and setting the "value" property of an HTMLInputElement writes to its <input> element's internal value. (The "value" attribute remains unchanged, since it is only used for initialization.) The DOM object is just modeling the actual element.
In general, the property on the DOM object will not exist unless it is specifically documented to, such as "id" and "value".
In JavaScript, you have an HTMLInputElement with a "value" property [1]. Getting the "value" property of an HTMLInputElement reads from its <input> element's internal value, and setting the "value" property of an HTMLInputElement writes to its <input> element's internal value. (The "value" attribute remains unchanged, since it is only used for initialization.) The DOM object is just modeling the actual element.
In general, the property on the DOM object will not exist unless it is specifically documented to, such as "id" and "value".
[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/in...
[1] https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputEl...