Try to use the title attribute, like this:

<button 
    id="checkout-now"
    type="button"
    class="action primary checkout"
    data-action="close"
    data-bind="attr: {href: checkoutNowUrl, title: checkoutNowTitle}"
/>

Here is example from the Magento_Authorizenet module:

app/code/Magento/Authorizenet/view/frontend/web/template/payment/authorizenet-directpost.html

<button data-role="review-save"
    type="submit"
    data-bind="
    attr: {title: $t('Place Order')},
    enable: (getCode() == isChecked()),
    click: placeOrder,
    css: {disabled: !isPlaceOrderActionAllowed()}
    "
    class="action primary checkout"
    disabled>
    <span data-bind="i18n: 'Place Order'"></span>
</button>

Try to use the title attribute and the text data-binding, like this:

<button 
    id="checkout-now"
    type="button"
    class="action primary checkout"
    data-action="close"
    data-bind="attr: {href: checkoutNowUrl, title: checkoutNowTitle}, text: checkoutNowTitle"
/>

Here is example from the Magento_Authorizenet module:

app/code/Magento/Authorizenet/view/frontend/web/template/payment/authorizenet-directpost.html

<button data-role="review-save"
    type="submit"
    data-bind="
    attr: {title: $t('Place Order')},
    enable: (getCode() == isChecked()),
    click: placeOrder,
    css: {disabled: !isPlaceOrderActionAllowed()}
    "
    class="action primary checkout"
    disabled>
    <span data-bind="i18n: 'Place Order'"></span>
</button>

Try to use the title attribute, like this:

<button 
    id="checkout-now"
    type="button"
    class="action primary checkout"
    data-action="close"
    data-bind="attr: {href: checkoutNowUrl}"
    translate="data-bind='attr: {title: checkoutNowTitle}'"
/>

Here is example from the Magento_Authorizenet module:

app/code/Magento/Authorizenet/view/frontend/web/template/payment/authorizenet-directpost.html

<button data-role="review-save"
    type="submit"
    data-bind="
    attr: {title: $t('Place Order')},
    enable: (getCode() == isChecked()),
    click: placeOrder,
    css: {disabled: !isPlaceOrderActionAllowed()}
    "
    class="action primary checkout"
    disabled>
    <span data-bind="i18n: 'Place Order'"></span>
</button>

Try to use the title attribute, like this:

<button 
    id="checkout-now"
    type="button"
    class="action primary checkout"
    data-action="close"
    data-bind="attr: {href: checkoutNowUrl, title: checkoutNowTitle}"
/>

Here is example from the Magento_Authorizenet module:

app/code/Magento/Authorizenet/view/frontend/web/template/payment/authorizenet-directpost.html

<button data-role="review-save"
    type="submit"
    data-bind="
    attr: {title: $t('Place Order')},
    enable: (getCode() == isChecked()),
    click: placeOrder,
    css: {disabled: !isPlaceOrderActionAllowed()}
    "
    class="action primary checkout"
    disabled>
    <span data-bind="i18n: 'Place Order'"></span>
</button>