After analyzing the hints provided in the suggested solutions, it becomes apparent that the rendering behavior observed in these instances may seem counterintuitive at first, but are actually correct and dictated by a combination of various rules outlined in the specification document and the overarching CSS box model.

1. CSS determines the necessary leading L for a box utilizing the formula line-height = L + AD, where AD is "the distance from the top to the bottom" of the font. This results in "half the leading added above A and the other half below D." Therefore, text with font-size:16px and line-height:24px will exhibit 4px of leading above and below. Text with font-size:8px and line-height:24px will display 8px of leading above and below.

2. By default, when specified as a percentage or em, user agents align glyphs ... by their relevant baselines. This sheds light on the observed phenomenon. If line-height is defined using percentage or em, the calculated value is inherited by the child element (in this case, the smaller span). Consequently, the smaller span inherits the parent block's line-height. However, due to the L + AD equation, the text within that span experiences more leading on top and bottom, resulting in the baseline being positioned higher within its box. The browser then vertically shifts the smaller span to align the baselines.

3. Subsequently, the browser encounters a new challenge — handling line spacing in the surrounding block disrupted by the baseline-altering process. The spec addresses this situation as well: the line-height of a block-level element "specifies the minimal height of line boxes within the element.". Therefore, CSS doesn't guarantee an exact line-height, only a minimum. As a result, the browser spaces out the lines in the enclosing block to accommodate the realigned child box.

The counterintuitive aspect stems from how most word processors and page-layout programs operate. In those applications, a smaller section of text within a paragraph aligns via its baseline (similar to CSS), while line height serves as the space between baselines rather than a box encapsulating the smaller text. Nevertheless, this isn't a flaw — CSS revolves around a box model. Thus, this spacing behavior can be attributed to that model.

However, we must still dissect the scenario involving unitless line-height:

1. It should be noted that in the absence of a specified line-height, browsers assign a unitless line-height by default. This is stipulated by the specifications: the initial value of line-height is normal, akin to "<number>", usually ranging between 1.0 and 1.2. This corresponds to what we observe in the aforementioned examples, where paragraphs with line-height: 1.5 display similar behavior to those without a specific line-height setting (implicitly adopting line-height: normal).

2. As mentioned by others, when the paragraph features line-height: 1.5, the calculated line-height of the paragraph does not transfer to the smaller span. Instead, the smaller span computes its own line height based on its font size. For instance, if the paragraph has

   line-height: 1.5; font-size: 14px

   , its calculated line height amounts to 14px * 1.5 = 21px. Conversely, if the smaller span only defines font-size: 50%, its font size equates to 14px * 50% = 7px, yielding a line height of 7px * 1.5 = 10.5px (usually rounded to a whole pixel). Essentially, the smaller box is half the size of the surrounding text.

3. Once again, the browser aligns the smaller span with the adjacent baseline. Nonetheless, since the box encompassing the smaller span is shorter than the neighboring text, this repositioning doesn't impact the enclosing block. It already accommodates the contents effortlessly, hence requiring no adjustment to spread the lines of the parent paragraph, unlike previously.

Both scenarios exemplify a unified interpretation of the specifications. This consistency allows for predictable prediction of line-spacing behavior.

This introspection redirects us to the initial query prompting this investigation. While understanding that the CSS box model necessitates this behavior, individuals like practicing typographers often find this approach less desirable. Ideally, one would prefer consistent & precise line spacing within paragraphs even when containing varying text sizes.

Regrettably, enforcing exact line spacing directly through CSS poses challenges compared to word processors or layout software. This limitation traces back to the CSS box model which doesn't adhere to baseline-to-baseline line-spacing norms; instead, line-height embodies a minimum measure and not maximum.

Nonetheless, embracing unitless line-height values emerges as the closest approximation to precise line spacing within CSS. Fervent typographers should feel reassured employing them, given their endorsement by the specs, delivering uniform outcomes across different browsers. They're neither a makeshift solution nor outdated.

An inherent limitation remains—a mere approximation. While unitless line-height values offer some semblance of control over line spacing, they don't revolutionize the core CSS box model nor positioning guidelines, potentially leading to unintended consequences in certain scenarios. Maintaining vigilance is imperative for pristine typography practices. Exercise caution.