console.log(getComputedStyle(document.getElementById("a")).getPropertyValue("--my-custom-property-1"))
console.log(getComputedStyle(document.getElementById("b")).getPropertyValue("--my-custom-property-2"))

#b-div { --my-custom-property-2: 'world' }

<div style="--my-custom-property-1: 'hello'"><h1 id="a">#a 'hello'</h1></div>
<div id="b-div"><h1 id="b">#b 'world'</h1></div>

function log(computed, selector, prop, value) {
  let method = computed ? "getComputedStyle(el)" : "el.style"
  let method_id = computed ? "computed" : "raw"

  // Build first level of list (tag name)
  let first = document.querySelector("#" + selector)
  if (!first) {
    first = document.createElement("li")
    first.appendChild(document.createTextNode(selector))
    first.setAttribute("id", selector)
    first.appendChild(document.createElement("ul"))
    document.querySelector("ul").appendChild(first)
  }

  // Build second level of list (method of style retrieval)
  let second = document.querySelector("#" + selector + "-" + method_id)
  if (!second) {
    second = document.createElement("li")
    second.appendChild(document.createTextNode(method))
    second.setAttribute("id", selector + "-" + method_id)
    second.appendChild(document.createElement("ul"))
    first.querySelector("ul").appendChild(second)
  }

  // Build third level of list (property accessed)
  let third = document.querySelector("#" + selector + "-prop" + prop)
  if (!third) {
    third = document.createElement("li")
    third.appendChild(document.createTextNode(prop + ": `" + value + "`"))
    third.setAttribute("id", "prop" + prop)
    second.querySelector("ul").appendChild(third)
    if (value === "") {
      third.classList.add("bad")
    } else {
      third.classList.add("good")
    }
  }
}

// Uses .style
function getStyleAttr(selector, prop) {
  let value = document.querySelector(selector).style.getPropertyValue(prop)
  log(false, selector, prop, value)
}

// Uses getComputedStyle()
function getStyleComputed(selector, prop) {
  let value = getComputedStyle(document.querySelector(selector)).getPropertyValue(prop)
  log(true, selector, prop, value)
}

// Loop through each property for each element and output the value
let selectors = ["article", "h1", "p"]
let props = ["--my-custom-property", "-my-custom-property"]
selectors.forEach(function(selector) {
  props.forEach(function(prop) {
    getStyleAttr(selector, prop)
    getStyleComputed(selector, prop)
  })
})

code {
  background: #eee;
  padding: .2em;
}

.bad {
  color: #800;
}

.good {
  color: #080;
}

<article class="custom-prop-inheritance" style="--my-custom-property: 'foobar'; -my-custom-property: 'foobar'">
  <h1>Title</h1>
  <p>Custom properties require two leading hyphens (<code>-my-custom-property</code> <em>never</em> works). Using <code>el.style</code> does not support inheritance. To support both inheritance and custom properties, you must use <code>getComputedStyle(<b>el</b>)</code> along with two leading hyphens on the custom property (eg, <code>--my-custom-property</code>).</p>
</article>
<ul></ul>