Overriding CSS Properties That Don't Have Default Values

Overriding CSS properties that don't have default values

If I'm reading your question correctly, do you want to, in one stylesheet, "erase" a declaration that you have in another stylesheet, such that the property will compute to the default value?

There's currently no way to reset it to whatever the value a browser uses as the default for a given element. The closest you can get is with the CSS3 initial keyword, which resets a property to its initial/default value according to the spec rather than according to how a browser defines it:

#element {
  left: initial;
  right: 0;
  top: 0;
}

There's not much browser support for it besides in Safari/Chrome and Firefox (as -moz-initial), so your next best alternative is to look up the initial value and hardcode it. For the left property, it's auto (and I believe it's this value for any element in all browsers anyway), so:

#element {
  left: auto;
  right: 0;
  top: 0;
}

Overriding CSS property all: unset

It does not appear to be possible to undo the effects of the all property once it has been specified. This may be due to all being a shorthand property (that happens to accept only the CSS-wide keywords as values).

You can't erase a shorthand declaration from the cascade the same way that css-cascade-4's introduction of the revert keyword allows you to erase author-level declarations, and that's because a shorthand property doesn't exist as its own entity in the cascade; instead, it simply represents all of its component properties. Like with the more traditional shorthand properties such as background and font, the only way to override a shorthand declaration that has been applied is to re-specify the values for the longhands that were overridden, either via longhand declarations or via another shorthand declaration. But you can't do the latter with the all property since it only accepts CSS-wide keywords.

As the former is obviously not practical with the all shorthand, since you can't predict which author-level declarations are being overridden to begin with, your only other option is to restrict it via a selector, thereby preventing it from ever applying in specific circumstances in the first place. Hopefully we will see more implementations of level 4 :not() in the near future, which will make writing selectors a little easier.

How can I nullify css property?

You have to reset each individual property back to its default value. It's not great, but it's the only way, given the information you've given us.

In your example, you would do:

.c1 {
    height: auto;
}

You should search for each property here:

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference

For example, height:

Initial value : auto

Another example, max-height:

Initial value : none

In 2017, there is now another way, the unset keyword:

.c1 {
    height: unset;
}

Some documentation: https://developer.mozilla.org/en-US/docs/Web/CSS/unset

The unset CSS keyword is the combination of the initial and inherit
keywords. Like these two other CSS-wide keywords, it can be applied to
any CSS property, including the CSS shorthand all. This keyword resets
the property to its inherited value if it inherits from its parent or
to its initial value if not. In other words, it behaves like the
inherit keyword in the first case and like the initial keyword in the
second case.

Browser support is good: http://caniuse.com/css-unset-value

How to reset/remove CSS styles for a specific element or selector only

The CSS property all has a keyword initial that sets the CSS property to the initial value as defined in the spec. The all keyword has broad browser support except for the IE and Opera Mini families.

/* basic modern patch */

#reset-this-root {
    all: unset;
}

#reset-this-root {
    all: initial;
}

Since IE's lack of support may cause issue here are some of the ways you can reset some CSS properties to their initial values:

.reset-this {
    animation : none;
    animation-delay : 0;
    animation-direction : normal;
    animation-duration : 0;
    animation-fill-mode : none;
    animation-iteration-count : 1;
    animation-name : none;
    animation-play-state : running;
    animation-timing-function : ease;
    backface-visibility : visible;
    background : 0;
    background-attachment : scroll;
    background-clip : border-box;
    background-color : transparent;
    background-image : none;
    background-origin : padding-box;
    background-position : 0 0;
    background-position-x : 0;
    background-position-y : 0;
    background-repeat : repeat;
    background-size : auto auto;
    border : 0;
    border-style : none;
    border-width : medium;
    border-color : inherit;
    border-bottom : 0;
    border-bottom-color : inherit;
    border-bottom-left-radius : 0;
    border-bottom-right-radius : 0;
    border-bottom-style : none;
    border-bottom-width : medium;
    border-collapse : separate;
    border-image : none;
    border-left : 0;
    border-left-color : inherit;
    border-left-style : none;
    border-left-width : medium;
    border-radius : 0;
    border-right : 0;
    border-right-color : inherit;
    border-right-style : none;
    border-right-width : medium;
    border-spacing : 0;
    border-top : 0;
    border-top-color : inherit;
    border-top-left-radius : 0;
    border-top-right-radius : 0;
    border-top-style : none;
    border-top-width : medium;
    bottom : auto;
    box-shadow : none;
    box-sizing : content-box;
    caption-side : top;
    clear : none;
    clip : auto;
    color : inherit;
    columns : auto;
    column-count : auto;
    column-fill : balance;
    column-gap : normal;
    column-rule : medium none currentColor;
    column-rule-color : currentColor;
    column-rule-style : none;
    column-rule-width : none;
    column-span : 1;
    column-width : auto;
    content : normal;
    counter-increment : none;
    counter-reset : none;
    cursor : auto;
    direction : ltr;
    display : inline;
    empty-cells : show;
    float : none;
    font : normal;
    font-family : inherit;
    font-size : medium;
    font-style : normal;
    font-variant : normal;
    font-weight : normal;
    height : auto;
    hyphens : none;
    left : auto;
    letter-spacing : normal;
    line-height : normal;
    list-style : none;
    list-style-image : none;
    list-style-position : outside;
    list-style-type : disc;
    margin : 0;
    margin-bottom : 0;
    margin-left : 0;
    margin-right : 0;
    margin-top : 0;
    max-height : none;
    max-width : none;
    min-height : 0;
    min-width : 0;
    opacity : 1;
    orphans : 0;
    outline : 0;
    outline-color : invert;
    outline-style : none;
    outline-width : medium;
    overflow : visible;
    overflow-x : visible;
    overflow-y : visible;
    padding : 0;
    padding-bottom : 0;
    padding-left : 0;
    padding-right : 0;
    padding-top : 0;
    page-break-after : auto;
    page-break-before : auto;
    page-break-inside : auto;
    perspective : none;
    perspective-origin : 50% 50%;
    position : static;
    /* May need to alter quotes for different locales (e.g fr) */
    quotes : '\201C' '\201D' '\2018' '\2019';
    right : auto;
    tab-size : 8;
    table-layout : auto;
    text-align : inherit;
    text-align-last : auto;
    text-decoration : none;
    text-decoration-color : inherit;
    text-decoration-line : none;
    text-decoration-style : solid;
    text-indent : 0;
    text-shadow : none;
    text-transform : none;
    top : auto;
    transform : none;
    transform-style : flat;
    transition : none;
    transition-delay : 0s;
    transition-duration : 0s;
    transition-property : none;
    transition-timing-function : ease;
    unicode-bidi : normal;
    vertical-align : baseline;
    visibility : visible;
    white-space : normal;
    widows : 0;
    width : auto;
    word-spacing : normal;
    z-index : auto;
    /* basic modern patch */
    all: initial;
    all: unset;
}

Relevant GitHub repo with a December 2017 more exhaustive list
Related
Related from MDN
Related W3C specs

With all this said, I don't think a CSS reset is something feasible unless we end up with only one web browser, if the 'default' is set by browser in the end.

Overriding styles without !important

It depends. CSS stands for Cascading Style Sheets, and there's a specific order that styles are applied in, overwriting previous styles. Without going into too much detail:

If your rules have the same specificity, just load your stylesheet second and everything will work fine.
If your rules have higher specificity, the order won't matter.
If your rules have lower specificity, you'll need to modify them to match.

So, what's specificity? Basically, it's the sum of each selector in a rule. So this:

a {
    background-color: black;
    color: white;
}

Has less specificity than this:

body a {
    color: orange;
}

ID selectors have higher specificity than class selectors, which have the same specificity as pseudo-class selectors, which have higher specificity than tag selectors. So if all your content is contained in a <div> with an id of content, you would be able to override a style that looks like this:

body a {
    border: 0;
}

With:

#content a {
    border: 1px solid black;
}

Reset CSS display property to default value

A browser's default styles are defined in its user agent stylesheet, the sources of which you can find here. Unfortunately, the Cascading and Inheritance level 3 spec does not appear to propose a way to reset a style property to its browser default. However there are plans to reintroduce a keyword for this in Cascading and Inheritance level 4 — the working group simply hasn't settled on a name for this keyword yet (the link currently says revert, but it is not final). Information about browser support for revert can be found on caniuse.com.

While the level 3 spec does introduce an initial keyword, setting a property to its initial value resets it to its default value as defined by CSS, not as defined by the browser. The initial value of display is inline; this is specified here. The initial keyword refers to that value, not the browser default. The spec itself makes this note under the all property:

For example, if an author specifies all: initial on an element it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.
This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. display: block from the UA style sheet on block elements such as <div>) will also be blown away.

So I guess the only way right now using pure CSS is to look up the browser default value and set it manually to that:

div.foo { display: inline-block; }
div.foo.bar { display: block; }

(An alternative to the above would be div.foo:not(.bar) { display: inline-block; }, but that involves modifying the original selector rather than an override.)

Override and reset CSS style: auto or none don't work

I believe the reason why the first set of properties will not work is because there is no auto value for display, so that property should be ignored. In that case, inline-table will still take effect, and as width do not apply to inline elements, that set of properties will not do anything.

The second set of properties will simply hide the table, as that's what display: none is for.

Try resetting it to table instead:

table.other {
    width: auto;
    min-width: 0;
    display: table;
}

Edit: min-width defaults to 0, not auto

Can I reset a CSS property rather than overriding it?

To answer your question (rather than solving your problem)...

Can I reset a CSS property rather than overriding it?

Reset to what?

The C in CSS stands for cascading and you'll always have several layers of styles combining among themselves with precisely defined though not always immediately clear rules. Apart from the styles set by the site author in different places (external CSS files, <style> blocks, style="" attributes...), in the base line we'll find the builtin browser styles and as far as I know browser vendors are free to assign whatever default styles they choose—and often users can add their own styles to the soup, either with builtin settings or with add-ons. Even the so called CSS resets don't actually reset anything. They merely add yet another layer of styles on top of the rest.

There's no syntax for "Create a snapshot here" (which would be the only solution I can think of without a thorough analysis) so the answer is basically no.

Overriding CSS Properties That Don't Have Default Values